[GWP-ASan] Various test fixes. (#94938)

When running some tests with --gtest_repeat=100 --gtest_shuffle, I
encountered some problems because the allocator wasn't torn down
completely, and the singleton pointer ended up pointing to a
use-after-scope'd object.

This patch has a couple of fixes and niceties:
 1. Removing the once-init stuff from tests, now that it's implicitly
    done in GuardedPoolAllocator::installAtFork() anyway.
 2. Calling uninitTestOnly() in the late_init test.
 3. Resetting the HasReportedBadPoolAccess when the signal handlers are
    installed (allowing for --gtest_repeat w/ recoverable mode).
 4. Adding a check and resetting the singleton pointer in
    uninitTestOnly().