Add all files from https://gitlab.com/zanoni-mbdyn/mbdyn-tests.git

Based on a request from @marco.morandini , all the "public" and "private" tests are included in MBDyn's main repository.

Now the following changes are applied:

• A test is allowed to fail if the following comment is added inside the input file:

## @MBDYN_SIMPLE_TESTSUITE_EXIT_STATUS@ = 1

• If a test cannot be executed within the testsuite, because it requires root permissions or communication with other software, the following comment must be added:

## @MBDYN_SIMPLE_TESTSUITE_EXCLUDE@ = 1

• In order to update the comment @MBDYN_SIMPLE_TESTSUITE_EXIT_STATUS@, the following command may be used:

testsuite/simple_testsuite.sh --update-reference-test-status {yes|no|all|failed|passed} --prefix-input tests

• All tests which failed, but were marked as @MBDYN_SIMPLE_TESTSUITE_EXIT_STATUS@ = 1 will be reported in the related merge request, but they will not cause the pipeline to fail. So, we could easily track the number of open issues.
• Almost all the unit tests are using gtest. If MBDyn was not compiled with support for gtest, then a built-in substitute for gtest is used.