build: fix up `make check` include path

* Add the tests/ directory itself to the list of include paths.
* .gitignore the produced gumbo_test binary and test runner logs.
test: link against system libgtest if available

Make ./configure check if libgtest is available system-wide and if so,
link against that. Before this commit, the `make check` target required
a checkout of libgtest in $(srcdir)/gtest - somewhat inconvenient.