[build] Address the System.Configuration test problem

The compiler seems to ignore reference assemblies with a full path when the
-lib: directive is passed, so things like:

      -lib:FOO -r:BAR/bar.dll

Would resolve bar.dll to FOO/bar.dll, instead of BAR/bar.dll.

This is a problem with the test suite as it needs to reference the
final build of System.Configuration from $(the_libdir_base) and not
the intermediate step.

To work around this issue, we introduce "test_remove" variable that is
a piece of text that will be removed from the TEST_COMPILE variable
when this is evaluated.

This means that it gets CSCOMPILE and then it removes the bits that we
are not interested in.