Tests: split the ExternalProject test

Move the subtests that test with local projects into their own test. The reason
is that on slower or crowded machines the test may reach the limit of 25
minutes and therefore fail while it would pass if it was given enough time.
The split is roughly 3:1 with regard to the execution time, with the new
ExternalProjectLocal test being the faster one.
Merge topic 'ExternalProject_test' into next

92af3664 Tests: split the ExternalProject test