test: seed the group a backward search miss has to clear - #7151

Follow-up to #7149, on a review comment it left behind.

#7149 added a test that asks what a backward search leaves in the match globals, ending with the miss: a search that finds nothing clears all of them, not $~ alone. The block seeded one group and read six names back, and two of the seven it means to cover were not covered.

$~ is the one the comment names, and no assertion asked for it. $2 was nil before the call, because "zzz" =~ /(z)/ never filled it, so assert_nil $2 would have passed on a clear that missed it just as well as on one that did not. Seeding /(z)(z)/ is what makes the assertion stand for the clear.

"zzz" =~ /(z)(z)/
"abc".rindex(/x/)   #=> nil
$~                  #=> nil
$2                  #=> nil, and it was "z" before the call

Every answer is CRuby's.

Verification

Dropping the clear from __regexp_rsearch, which is the mistake the block is there to catch, turns 7 of its assertions red where it turned 5 before, $~ and $2 among them.

Full suite green on build_config/ci/gcc-clang.rb: full-core with MRB_GC_STRESS (2286), bintest (2286), the C++ ABI (2285), and the default gembox (2070). 0 failures, 0 crashes, no new compiler warnings.

Summary by CodeRabbit

• Bug Fixes
  • Improved regular-expression backward-search behavior by ensuring all match results are cleared after an unsuccessful search.
  • Added coverage for multiple capture groups to prevent stale match data from being retained.