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 everywhere.
Some tests cannot be run individually, for example ./test/t8_cmesh/t8_gtest_cmesh_set_join_by_vertices_serial gives the error Failed to open test/testfiles/test_cube_unstructured_1.inp.

The test files are currently copied to a directory called build/test/test/testfiles. When I place them in a build/test/testfiles directory, the individual tests run as expected, but the general test suite fails.

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

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