When using `-s` to specify a particular test, it will do a prefix match.
Thus, `-sapply::both::rename_a_to_b_to_c` will match both a test named
`test_apply_both__rename_a_to_b_to_c` and a test that begins with that
name, like `test_apply_both__rename_a_to_b_to_c_exact`.

Permit a trailing `$` to `-s` syntax.  This allows a user to specify
`-sapply::both::rename_a_to_b_to_c$` to match _only_ the
`test_apply_both__rename_a_to_b_to_c` function.

We already filter to ensure that the given prefix matches the current
test name.  Also ensure that the length of the test name matches the
length of the filter, sans trailing `$`.