Some GPA tests are disabled on WASI

[mlugg]

In #20271 (which I expect to be merged imminently), I have had to disable some std.heap.GeneralPurposeAllocator tests on wasm32-wasi (no libc) because they started failing for no clear reason. These tests run without crashing, but end up reporting leaks.

• The failure consistently reproduces locally when running the whole standard library tests.
• It does not reproduce if the failing tests are extracted to their own file.
• It does not reproduce when filtering for only the GPA tests or only a failing test.

Since the repro seems to care about the rest of the test binary, it seems like #20271 has just exposed a pre-existing bug, particularly since that PR doesn't change anything which could reasonably affect GPA.