fix(cli/run.js): Revert default glob to match Mocha-5.2

The old glob 'test' would allow 'test.js' to be used by default (rather than requiring file be
placed inside "test" directory.

Fixes #3750

lib/cli/run.js

@@ -245,7 +245,7 @@ exports.builder = yargs =>
       }
     })
     .positional('spec', {
-      default: ['test/'],
+      default: ['test'],
       description: 'One or more files, directories, or globs to test',
       type: 'array'
     })