Ignore tests using System.setSecurityManager (bp #1661) (#1664)

* Ignore tests using System.setSecurityManager (#1661)

The SecurityManager is global so is not thread-safe. This is the source
of flaky tests in FIRRTL CI.

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>
(cherry picked from commit 9f1d6cbb

)

# Conflicts:
#	src/test/scala/chiselTests/stage/ChiselStageSpec.scala

* Resolve merge conflicts

Co-authored-by: Jack Koenig <koenig@sifive.com>