Bump mutex_m from 0.1.2 to 0.2.0 in /spec/tester_mongo

Bumps [mutex_m](https://github.com/ruby/mutex_m) from 0.1.2 to 0.2.0.
- [Release notes](https://github.com/ruby/mutex_m/releases)
- [Commits](ruby/mutex_m@v0.1.2...v0.2.0)

---
updated-dependencies:
- dependency-name: mutex_m
  dependency-type: indirect
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

spec/tester_mongo/Gemfile.lock

@@ -134,7 +134,7 @@ GEM
       mongo (>= 2.18.0, < 3.0.0)
       ruby2_keywords (~> 0.0.5)
     msgpack (1.7.2)
-    mutex_m (0.1.2)
+    mutex_m (0.2.0)
     net-imap (0.4.4)
       date
       net-protocol