test/ruby/test_m17n.rb: Run a test under assert_separately

The test uses `Encoding.default_external = Encoding::UTF_16BE`, which
may add a wrongly UTF_16BE-encoded path to $LOADED_FEATURES (depending
on the order of tests). Unfortunately this breaks another test:

http://ci.rvm.jp/results/trunk-test@ruby-sky1/3711615
```
/tmp/ruby/v3/src/trunk-test/test/io/console/test_io_console.rb:11:in `===': incompatible encoding regexp match (US-ASCII regexp with UTF-16BE string) (Encoding::CompatibilityError)
```

According to @naruse-san, we don't pay effort to such a case, so this
change just avoids the issue by running the test in question under
another process.

Co-Authored-By: Koichi Sasada <ko1@atdot.net>

0 files changed, 0 insertions, 0 deletions