test: update travis config, handle errors properly, add TEST_LLNODE_DEBUG

[joyeecheung]

At the moment when a test times out (usually because the expected lldb output does not show up, possible due to an error), there is only a tape timeout message, as is what's happening in our Travis CI build.

This PR checks the timeout and throws an error containing the lldb output for the ease of debugging.

I've tried it on Mac and Linux with v8.9.0 and v6.11.5. I've noticed that the tests are a bit flaky in master, with random timeouts (with this patch on I can see the sent command got swallowed, it's 8 <command> instead of v8 <command> so lldb will error and the test times out). Sometimes there is a ECONNRESET on the pipe. I'll investigate that later.

[joyeecheung]

The Travis builds now error instead of timing out. There is a reproduction of the "swallowed v": https://travis-ci.org/nodejs/llnode/jobs/296685259. Others seems to stuck in v8 bt.

[joyeecheung]

Just noticed that since we only push the stdout lines into the buffer, the stderr lines are not available...for example on linux where process save-core is not available, the line error: Failed to save core file for process: no ObjectFile plugins were able to save a core for this process is written into stderr, not stdout.

Need to rework this a bit to show stderr lines when timing out as well.

[joyeecheung]

Travis is now green (see https://travis-ci.org/nodejs/llnode/builds/308579054), although sometimes it fails because of #150 .

This is now ready for review, cc @cjihrig@indutny@bnoordhuis@hhellyer , thanks!

[joyeecheung]

@cjihrig Added a containsLine function to DRY the usage-test. The process.version comparison can be fixed in a later PR.

If there are no more reviews in 2 days I would like to land this so we can get a green Travis CI. The jenkins one probably need a bit of configuration update (last time I tried it it was using the old flow to build the plugin and failed to build it).