mruby-regexp: test break out of the Regexp#match block - #6992

Regexp#match accepts a block (MRB_ARGS_BLOCK() in regexp.c, yielded in
regexp_match()), but the suite only covered the value the block returns
normally:

assert("Regexp#match - block") do
  result = /bc/.match("abcd") { |md| [md[0], md.begin(0)] }
  assert_equal ["bc", 1], result
  assert_nil(/xyz/.match("abcd") { |md| md[0] })
end

Nothing exercised a non-local exit from that block. The behaviour is correct
today, because mrb_yield() propagates the break:

$ ./build/host/bin/mruby -e 'p(/l+/.match("hello") { break :broke })'
:broke

That is an easy property to lose when regexp_match() is refactored -- for
example if the yield moves behind an error-handling wrapper, or is replaced by
a call that swallows the break state. The regression would be silent: the
method would return the MatchData instead of the break value.

Test-only; no implementation change.

Related to #6991, which adds the same coverage for String#match once the
block is forwarded there. This one stands on its own and touches a different
part of the test file, so the two do not conflict.

rake test passes.

Summary by CodeRabbit

• Bug Fixes

  • Fixed Regexp#match to return the value provided by a block when it exits early with break.

• Tests

  • Added regression coverage for early block exits.