Unit tests: Unreachable block in tests::test_main (panic hook)

This isn't a compiler bug. But a "memory leak" in the unit tests main function that valgrind catches.
It's not a memory leak in the strict sense but a reachable block we never free and shows up in Valgrind summary.
I think it's better user experience if it won't be reported.

To reproduce you can do the following:

❯ cargo new reachable_block_with_cargo_test --lib
     Created library `reachable_block_with_cargo_test` package

gelbard in 🌐 ~/Projects took 41s
❯ cd reachable_block_with_cargo_test/

gelbard in 🌐 ~/Projects/reachable_block_with_cargo_test on  master [?]
❯ cargo test
   Compiling reachable_block_with_cargo_test v0.1.0 (/home/gelbard/Projects/reachable_block_with_cargo_test)
    Finished test [unoptimized + debuginfo] target(s) in 0.27s
     Running unittests src/lib.rs (target/debug/deps/reachable_block_with_cargo_test-912744a325bf7c5e)

running 1 test
test tests::it_works ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

   Doc-tests reachable_block_with_cargo_test

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s


gelbard in 🌐 ~/Projects/reachable_block_with_cargo_test on  master [?] took 22s
❯ /snap/bin/valgrind --leak-check=full --show-leak-kinds=all target/debug/deps/reachable_block_with_cargo_test-912744a325bf7c5e
==146594== Memcheck, a memory error detector
==146594== Copyright (C) 2002-2022, and GNU GPL'd, by Julian Seward et al.
==146594== Using Valgrind-3.22.0 and LibVEX; rerun with -h for copyright info
==146594== Command: target/debug/deps/reachable_block_with_cargo_test-912744a325bf7c5e
==146594==

running 1 test
test tests::it_works ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.09s

==146594==
==146594== HEAP SUMMARY:
==146594==     in use at exit: 16 bytes in 1 blocks
==146594==   total heap usage: 446 allocs, 445 frees, 61,882 bytes allocated
==146594==
==146594== 16 bytes in 1 blocks are still reachable in loss record 1 of 1
==146594==    at 0x4A390C5: malloc (vg_replace_malloc.c:442)
==146594==    by 0x151336: alloc (alloc.rs:98)
==146594==    by 0x151336: alloc_impl (alloc.rs:181)
==146594==    by 0x151336: allocate (alloc.rs:241)
==146594==    by 0x151336: exchange_malloc (alloc.rs:330)
==146594==    by 0x151336: new<test::test_main::{closure_env#0}> (boxed.rs:217)
==146594==    by 0x151336: test::test_main (lib.rs:124)
==146594==    by 0x1522F9: test::test_main_static (lib.rs:160)
==146594==    by 0x11E102: reachable_block_with_cargo_test::main (lib.rs:1)
==146594==    by 0x11EABA: core::ops::function::FnOnce::call_once (function.rs:250)
==146594==    by 0x11E76D: std::sys_common::backtrace::__rust_begin_short_backtrace (backtrace.rs:154)
==146594==    by 0x11DFC0: std::rt::lang_start::{{closure}} (rt.rs:166)
==146594==    by 0x177D3A: call_once<(), (dyn core::ops::function::Fn<(), Output=i32> + core::marker::Sync + core::panic::unwind_safe::RefUnwindSafe)> (function.rs:284)
==146594==    by 0x177D3A: do_call<&(dyn core::ops::function::Fn<(), Output=i32> + core::marker::Sync + core::panic::unwind_safe::RefUnwindSafe), i32> (panicking.rs:504)
==146594==    by 0x177D3A: try<i32, &(dyn core::ops::function::Fn<(), Output=i32> + core::marker::Sync + core::panic::unwind_safe::RefUnwindSafe)> (panicking.rs:468)
==146594==    by 0x177D3A: catch_unwind<&(dyn core::ops::function::Fn<(), Output=i32> + core::marker::Sync + core::panic::unwind_safe::RefUnwindSafe), i32> (panic.rs:142)
==146594==    by 0x177D3A: {closure#2} (rt.rs:148)
==146594==    by 0x177D3A: do_call<std::rt::lang_start_internal::{closure_env#2}, isize> (panicking.rs:504)
==146594==    by 0x177D3A: try<isize, std::rt::lang_start_internal::{closure_env#2}> (panicking.rs:468)
==146594==    by 0x177D3A: catch_unwind<std::rt::lang_start_internal::{closure_env#2}, isize> (panic.rs:142)
==146594==    by 0x177D3A: std::rt::lang_start_internal (rt.rs:148)
==146594==    by 0x11DF99: std::rt::lang_start (rt.rs:165)
==146594==    by 0x11E12D: main (in /home/gelbard/Projects/reachable_block_with_cargo_test/target/debug/deps/reachable_block_with_cargo_test-912744a325bf7c5e)
==146594==
==146594== LEAK SUMMARY:
==146594==    definitely lost: 0 bytes in 0 blocks
==146594==    indirectly lost: 0 bytes in 0 blocks
==146594==      possibly lost: 0 bytes in 0 blocks
==146594==    still reachable: 16 bytes in 1 blocks
==146594==         suppressed: 0 bytes in 0 blocks
==146594==
==146594== For lists of detected and suppressed errors, rerun with: -s
==146594== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

Reproduce the report with a generated library using cargo test, then run its test binary under Valgrind. Inspect test::test_main at lib.rs:124 and test::test_main_static at lib.rs:160; done means the test harness no longer leaves the reported reachable block while still passing its tests.