Make global-showdef a DirectTest

The test test/files/run/global-showdef.scala was outputting
to the cwd instead of the test output dir.

Good behavior is now inherited from DirectTest.

Test frameworks, of any ilk or capability, rock.
Reduce compiles for global-showdef test

You can only show one class or object at a time,
but we can show one of each to reduce the compilations
for this test.

It seems the original issue happened because the test
started to create class files after SI-8217.

So, also stop compile after typer, because why stress the kitteh.
Merge pull request #4063 from som-snytt/issue/direct-showdef-test

Make global-showdef a DirectTest