Correct TST_SRC lib path, lazy load grunt-mocha-test

TST_SRC path for `test\lib` had an extra directory level.
Removed unused `path` require.
Lazy load `grunt-mocha-test` following comment [here](https://github.com/gruntjs/grunt/issues/975#issuecomment-29058707). While there isn't a huge performance gain, a custom task for mocha already existed, so might as well move one line of code and save a few ms. I used `time-grunt` to prove that it does make a small difference (just under 30ms) when running jshint for example.

Before change:
Execution Time (2013-12-19 14:43:48 UTC)
loading tasks  178ms  ▇▇▇▇▇▇ 11%
jshint:tests   224ms  ▇▇▇▇▇▇▇ 13%
jshint:src        1s  ▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇ 76%
Total 2s

After:
Execution Time (2013-12-19 14:46:22 UTC)
loading tasks  151ms  ▇▇▇▇▇ 9%
jshint:tests   225ms  ▇▇▇▇▇▇▇ 14%
jshint:src        1s  ▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇ 77%
Total 2s