test: Use mocha-clean

  This module cleans up stack traces of errors raised in tests run with
  mocha.
  It was already set as a dev dependency but was not used.

.mocharc.json

@@ -7,6 +7,7 @@
     "./core/globals",
     "should",
     "should-sinon",
+    "mocha-clean",
     "./test/support/assertions/change",
     "./test/support/assertions/fileContents",
     "./test/support/assertions/pending",