Add performance testing framework: preparatory refactoring

[tamiko]

@peterrum and I are currently working on bringing back performance tests to deal.II. Our idea is to integrate performance tests as a part of the testsuite with the following rationale:

• By default performance tests will be compiled and linked but do not run (with expected stage RUN or DIFF). This will ensure that compilation and linking of performance test code is part of our CI and does not diverge from the library API.
• We will use ctest (and our testsuite infrastructure) to compile and run performance tests that simply report (a best of three) timing result to the output file. These output files can then further be postprocessed by a script to create a table. All of this will be chained together in a run_performance_tests.cmake ctest script.

This PR adds some necessary features to the current testsuite infrastructure:

• implement thread concurrency annotation via [...].threads=XY.output
• the ability to set MPI rank and thread limits via TEST_THREAD_LIMIT and TEST_MPI_RANK_LIMIT. Tests that specify a higher number of MPI ranks or thread concurrency will be dropped.
• allow to use maximal concurrency by setting .threads=max. or .mpirun=max..

[tamiko]

/rebuild

[peterrum]

Looks good to me! Thanks for making the first steps!

Since I am too much involved here, I would appreciate a second opinion!

[masterleinad]

Looks good to me.

[peterrum]

Let's merge. More is coming!