rm: consistent naming of test functions

[cakebaker]

This PR does three trivial things:

• replaces the test_rm_ prefix used for some test function names with test_ for consistent naming
• simplifies some tests by using new_ucmd!
• merges some imports

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/tee (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/timeout/timeout (passes in this run but fails in the 'main' branch)