Follow-up to #47942.

This commit simplifies the normalization of regexp test filters.
Instead of modifying a given regexp to match a union of whitespace and
underscores, the regexp is unioned with the underscore-normalized
version of itself.  This allows filters that include escaped spaces,
such as `/foo\ bar/`.

This commit also fixes `Rails::LineFiltering#run` such that
normalization isn't reapplied for every test suite.  Thus
`normalize_declarative_test_filter` is no longer required to be
idempotent.

Co-authored-by: Jonathan Hefner <jonathan@hefner.pro>