[gdb/testsuite] Set TSAN_OPTIONS by default to history_size=7

[gdb/testsuite] Set TSAN_OPTIONS by default to history_size=7

I build gdb with -fsanitize=thread and ran the testsuite, and ran into the
case that a race is detected, but we see the full stack trace only for one of
the two accesses, and the other one is showing "failed to restore the stack".

Try to prevent this by setting ThreadSanitizer flag history_size [1] to the
maximum (7) by default, as suggested here [2].

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>
[1] https://github.com/google/sanitizers/wiki/ThreadSanitizerFlags
[2] https://groups.google.com/g/thread-sanitizer/c/VzSWE7UxhIE