Tests fail if build directory is outside repo

[lsawade]

The bug

A lot of the tests require files that need to be read as part of the test execution. E.g. specfem_config.yml files. The paths are defined as relative paths from specfempp/build/tests/unit-tests/ to specfempp/tests/unit-tests/ and will therefore fail if the tests are built in e.g.
specfempp/../build/.

Solution

One solution is to use CMake to actually manage test running through the add_test() directive, and define a working directory for each test:

add_executable(testA testA.cpp)
add_test(NAME ThisIsTestA COMMAND testA WORKING_DIRECTORY ${DIRECTORY_WITH_TEST_DATA})