[tsan] Fix nested signal handling (#138599) This PR fixes the bug reported in #134358. In the current implementation of the tsan posix interceptors, the signal set does not get restored to the correct original set, if a signal handler gets called, while already inside of a signal handler. This leads to the wrong signal set being set for the thread in which the signal handler was called. To fix this I introduced a stack of `__sanitizer_sigset_t` to keep all the correct old signal sets and restore them in the correct order. There was also already an existing test that tested nested / recursive signal handlers, but it was disabled. I therefore reenabled it, made it more robust by waiting for the second thread to have been properly started and added checks for the signal sets. This test then failed before the introduction of the interceptor fix and didn't fail with the fix. @dvyukov What are your thoughts?
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