haxe: Fix path to stdlib and add small test

The version bump in c727e7e7

 (pull
request #35153) didn't actually take into account that Haxe has changed
the way they search for the stdlib. Instead of a hardcoded list of paths
it now searches based on a common prefix.

So when running Haxe, it errored out because it couldn't find its own
standard library. This is now fixed by changing the sed expression
accordingly.

Apart from fixing the actual issue, I've added a small test in
installCheckPhase to make sure something like this won't happen again in
future updates.

Signed-off-by: aszlig <aszlig@nix.build>
Cc: @volth, @joachifm