Disable gtkdoc-check

It does not get along with automake's new parallel test runner:

  fatal: making test-suite.log: failed to create /usr/bin/gtkdoc-check.trs
  fatal: making test-suite.log: failed to create /usr/bin/gtkdoc-check.log

It was not actually doing anything anyway, as we set

  TESTS_ENVIRONMENT = cd $(top_srcdir)

(gtkdoc-check needs to be run in docs/reference)

Also, there are some errors which we do not want to fix, at least right now.
gitignore: Ignore *.log and *.trs files

These are created by automake's parallel test runner.
test-umockdev-run: Drop hardcoded path to src/

Set it in test environment instead, so that we can also run it against the
system installed packages.
Add "make check-installed" target

This runs tests against the installed umockdev instead of against the build
tree.