Tests for dynamic regexes will now run during 'check-stage2'.

Before, tests for dynamic regexes ran during stage1 and tests for
native regexes ran during stage2. But the buildbots don't test stage1,
so now both dynamic and native tests are run during stage2.

Closes #13740.
auto merge of #13766 : BurntSushi/rust/regex-dynamic-tests, r=alexcrichton

Before, tests for dynamic regexes ran during stage1 and tests for
native regexes ran during stage2. But the buildbots don't test stage1,
so now both dynamic and native tests are run during stage2.

Closes #13740.