Refactor the Plugin test.

Policy CMP0024 was introduced to disallow the include() of a file
generated by export().

Use ExternalProject to test the plugins after building.
Make ExportLanguages a subtest of the ObjectLibrary test

It was a subtest of the RunCMake.ObjectLibrary test. However, we need
to test a build with ExternalProject after running CMake, which RunCMake tests
do not do.
Merge topic 'handle-CMP0024-in-tests' into next

310aef9 Make ExportLanguages a subtest of the ObjectLibrary test
30ff6cf Refactor the Plugin test.