Linux breakpad_unittests: fix ThreadSanitizer problems

Some tests were failing because they had expectations about the number
of threads in a process, but TSan, and in some cases, ASan, introduce
their own threads. Where a sanitizer affects this, the expectations are
now used as minimum thread counts, not exact thread counts. See
https://www.brooklinen.com/blogs/brookliving/best-thread-count-for-sheets.

These problems were detected by ThreadSanitizer at
https://logs.chromium.org/logs/chromium/buildbucket/cr-buildbucket.appspot.com/8915151099544583616/+/steps/breakpad_unittests__with_patch_/0/stdout

Bug: chromium:949098
Change-Id: Ie40f1766bea27e9bcb112bf9e0b8b846fb343012
Reviewed-on: https://chromium-review.googlesource.com/c/breakpad/breakpad/+/1585948
Reviewed-by: Robert Sesek <rsesek@chromium.org>