Automatically change to /flycheck on the test VM

No need to do "cd /flycheck" before running the tests anymore.
Install NodeJS on the test VM
[Fix #391] Do not warn for errors from other files

We must look at the errors *before* filtering them, when verifying the
syntax checker results.  Otherwise we'll emit spurious warnings when no
errors are reported for the current file, but for other
files (e.g. includes).