Add TEST_REQUIRES keyword in ecbuild_add_test

[tweska]

This change adds a new keyword TEST_REQUIRES which can be used to indicate dependencies between tests. When you run a single test, all the dependencies of that test will also run. This is very useful for any test that requires some data from nexus. Another good example is the tests in MultIO that compare the results of another test against a reference file.

This works by setting the FIXTURES_SETUP on the dependency, and FIXTURES_REQUIRED on the test that has the dependency.

If you do not use the new keyword anywhere, the fixtures will not be added anywhere in your project either (unless you add it explicitly yourself). And, since it is a new keyword, this should be fully backwards compatible with existing cmake files.

I added two tests, one that does a very simple chain of dependencies where every test depends on the test before it. The other test has one test which depends on two tests, the dependency graph has a diamond shape.

[marcosbento]

Looks like a good idea. I'll approve to let the tests run, and will eventually proceed with a code review.

[wdeconinck]

This looks great! Thanks @tweska !

[marcosbento]

Looks good to me!
Thank you for the contribution, @tweska.
Merging now.