v test reporter crashes with memory allocation failure (heap corruption)

V version

V 0.5.2 aca60a4 (master HEAD, 2026-09-17). The weekly.2026.08 prebuilt
runs the same suite green.

Repro

Any multi-test _test.v file of reasonable size, e.g. the
audit_checks_test.v in HorneroOS/hornero (main branch,
cli/modules/hornero_core/, run with VMODULES=$PWD/modules v test ./modules/hornero_core/audit_checks_test.v):

---- Testing... ----------------------------------------------------------------
malloc(0x3f94c34e73700)
V panic: memory allocation failure
 v hash: aca60a4
src.c:20170: at v_panic: Backtrace
src.c:7564: by _memory_panic
src.c:20141: by v_malloc
src.c:10412: by new_dense_array
src.c:7455: by VMapData__set
src.c:10243: by map__set
src.c:17228: by testing__NormalReporter__report
src.c:5822: by testing__Reporter__report
src.c:17777: by testing__TestSession__print_messages

The crash is inside the test reporter itself (map__set while printing
test messages), before any test result is shown. The same tree compiled
with the weekly toolchain passes fully. Single tiny test files pass;
the corruption needs a larger suite, and manifests nondeterministically
(sometimes a hang instead of the panic).

Related: #28692 (same class of memory corruption in v vet's
diagnostic strings on the same toolchain).

Expected

Test reporter prints results instead of corrupting the heap.

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Reproduce with VMODULES=$PWD/modules v test ./modules/hornero_core/audit_checks_test.v and compare the current toolchain with the weekly.2026.08 build. Start at testing__NormalReporter__report in the stack trace, where map__set fails while printing test messages; use the single-test and larger-suite cases to narrow the corruption. Done means the multi-test file prints its results without a panic or hang.