Add a test to check that build-tool-depends are used (#10692)

The testcase is not so easy to write because

* The bug only surfaces when the build-tool you are depending on is
  known (ie alex, happy etc)
* But then it is tricky to write a test, as we can't depend on the known
  tools or bundle the source for them.
* So we create a fake "alex", which cabal then invokes on a fake ".x"
  file. This is maybe a bit fragile if the way cabal invokes alex
  changes in future, but then the test can be modified as well.

Ticket #10692