tests: Removed m5threads tests from .testignore

tests: Removed m5threads tests from .testignore

This commit fixes many problems which were resulting in these tests
not executing correctly. However, the m5thread tests are still failing
with an `fatal:syscall set_tid_address (#166) unimplemented` error,
recorded here: https://gem5.atlassian.net/browse/GEM5-747.

The tests have been removed from .testignore as part of our goal of
removing all tests from the .testignore file:
https://gem5.atlassian.net/browse/GEM5-361

Change-Id: I287d1e126963114a791d7f3aa563a037a89b2cb7
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/32916
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>