Use path to testfiles instead of copying testfiles to build directory

[lenaploetzke]

Feature request

Currently, the folder test/testfiles is copied into the build folder when building the tests.
This is not necessary, you can also set the path to the test files in cmake.
This is especially a problem if some of the testfiles get big.
If we use the paths solution, the tests does not work anymore if the testfiles in the main repository are deleted.
The current solution works and is simple.

Update:
I have noticed that the current solution does not work very well.
Individual test binaries using testfiles has to be executed from the build/test folder to be successful as the paths are set accordingly. For example ./test/t8_cmesh/t8_gtest_cmesh_set_join_by_vertices_serial executed from the build folder gives the error Failed to open test/testfiles/test_cube_unstructured_1.inp.

I expect that the solution described above will also solve this problem.

Estimated priority
"Priority: medium" Should be solved within half a year