Fixed test framework involving tests that import external libraries

This submission fixes tests that import libraries such as ```FFITarget.dll``` and use them for example those in ```LibraryTests``` and ```CodeBlockCompletionTests```. The reason for the regressions due to these tests was that a static instance of ```DynamoPathManager``` maintained a list of ```PreloadedLibraries``` from a previous test run, which was not cleared at the end of the run due to which the previously loaded library (in this case, ```FFITarget.dll```) was reloaded in the subsequent test case run. The fix addresses this by destroying the static instance at ```Cleanup``` (see ```DestroyInstance```) of each test case so that at the start of the next test case, a new one can be instantiated.

@Benglin, @sharadkjaiswal could you take a look?
Can we make this method internal?