add test for FlatIO port ordering (backport #4113) (#4114)

* Add test for FlatIO port ordering (#4113)

(cherry picked from commit c3c99793

)

# Conflicts:
#	src/test/scala/chiselTests/experimental/FlatIOSpec.scala

* Added MatchesOrOmits

* Resolve backport conflicts

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Co-authored-by: Megan Wachs <megan@sifive.com>
Co-authored-by: Adam Izraelevitz <azidar@gmail.com>
Co-authored-by: Jack Koenig <koenig@sifive.com>