derive the path of Paths_haskell_awk

While the path is often "dist/autogen/Paths_haskell_awk", some cabal
commands build in a separate folder, causing the tests to fail.
Fortunately, we can figure out where this separate folder is by looking
at the path of the running executable.
install before testing

see https://github.com/gelisam/install-before-test for details.
fix "cabal test"

The fix to get tests to run during "cabal install --enable-tests" broke
the simpler command "cabal test". By forcing an install, we were
forwarding to "cabal install" arguments meant for "cabal test".
Merge branch 'issue-107', closes #107

The title of the issue is "regression: cabal test should warn about
missing runtime", but the chosen solution was to force the runtime
to be installed any time the tests are ran.