runtime:mayMoreStackPreempt: TestScanAllocIssue77573/heap_slice_([]*int,_3) failures

```
#!watchflakes
default <- pkg == "runtime:mayMoreStackPreempt" && test == "TestScanAllocIssue77573/heap_slice_([]*int,_3)"
```

Issue created automatically to collect these failures.

Example ([log](https://ci.chromium.org/b/8673402588962897617)):

=== RUN TestScanAllocIssue77573/heap_slice_([]*int,_3)
malloc_test.go:872: wrong GC Scan Alloc Size:
want 24
got 16
--- FAIL: TestScanAllocIssue77573/heap_slice_([]*int,_3) (0.00s)

— [watchflakes](https://go.dev/wiki/Watchflakes)

Start by running TestScanAllocIssue77573/heap_slice_([]*int,_3) and inspecting malloc_test.go around line 872, where the wrong GC Scan Alloc Size is reported. Use the linked failure log to compare the expected 24 bytes with the observed 16 bytes. Done means this subtest no longer reports the scan allocation size mismatch.

Written by the indexing model from the issue text.