mocha_test: add mocha to runfiles

[ixdy]

Something changed in Bazel 0.5.0 such that this rule stopped working, e.g.:

module.js:457
    throw err;
    ^

Error: Cannot find module '/root/.cache/bazel/_bazel_root/27a95fe2fc68aa1195ddf1a4bbaaea62/bazel-sandbox/2986611709447846617/execroot/test-infra/bazel-out/local-fastbuild/bin/triage/script_test.runfiles/__main__/t/.cache/bazel/_bazel_root/27a95fe2fc68aa1195ddf1a4bbaaea62/external/npm_mocha/bin/mocha'
    at Function.Module._resolveFilename (module.js:455:15)
    at Function.Module._load (module.js:403:25)
    at Module.runMain (module.js:590:10)
    at run (bootstrap_node.js:394:7)
    at startup (bootstrap_node.js:149:9)
    at bootstrap_node.js:509:3
Use --strategy=TestRunner=standalone to disable sandboxing for the failing actions.

I don't exactly understand what _get_abs_sourcepath is doing, but it was still getting confused, even after adding mocha to the runfiles. Using just mocha.path seems to work as intended, though, and is far simpler.

cc @rmmh

[ixdy]

Seems like node_library is still broken. manually parsing the File object (looking for Artifact:) seems to be problematic.