remove duplicate 'test' from mocha args
remove duplicate 'test' from mocha args

f620fae added to mocha's command line arguments to include the 
node-test directory. However, it also added a duplicate argument for
the test directory. This commit removes the duplicate test
josdejong/mathjs,FSMaxB/mathjs,josdejong/mathjs,josdejong/mathjs,FSMaxB/mathjs,FSMaxB/mathjs,josdejong/mathjs,FSMaxB/mathjs,josdejong/mathjs,FSMaxB/mathjs