Ignore errors when installing syntax checker tools

We should rather skip tests than fail on the entire test suite if a
single checker tool is currently unavailable.

In light of the recent search.cpan.org outages…
Remove redundant file
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).
Add option to set Haskell language extensions for GHC

In preparation for flycheck/flycheck-haskell#3
Merge pull request #392 from flycheck/ghc-extensions

Add option to set Haskell language extensions for GHC
Add Go errcheck checker