mruby-bin-mruby: skip regexp split bintest when mruby-regexp absent

The bintest added in 9d8d41006b uses `"hello world".split(/\s+/)`,
which raises `uninitialized constant Regexp` in builds that omit
mruby-regexp. Probe whether the mruby binary defines Regexp and
skip the whole assert block when it doesn't; the test is regression
coverage for mruby-regexp's String#split override, so when regexp
isn't loaded there is nothing to verify.

close #6832

Co-authored-by: Claude <noreply@anthropic.com>

mrbgems/mruby-bin-mruby/bintest/mruby.rb

@@ -178,6 +178,10 @@ assert('top level local variables are in file scope') do
 end
 
 assert('String#split still works when mruby-regexp is loaded') do
+  # Only meaningful when mruby-regexp is built in; skip otherwise.
+  _, _, stat = Open3.capture3(*(cmd_list("mruby") + ["-e", "Regexp"]))
+  skip "mruby-regexp not loaded" unless stat.success?
+
   # The regexp-aware override in mruby-regexp/mrblib/string_regexp.rb used to
   # replace the C-defined String#split, leaving its `return super if ...`
   # fast paths with no method to delegate to (NoMethodError).  Now the