Use distinct file names

[certik]

This seems to avoid a filename race conditions on macOS.

Fixes #264.

[certik]

The actual problem seems to be that the ctest tests are executed in parallel, and the tests in test_stdlib_bitset_64 and in test_stdlib_bitset_large where both reading/writing to test.txt. That is definitely a bug, that was fixed.

However, even after renaming the test file in test_stdlib_bitset_large, the test test_stdlib_bitset_64 failed, even though I didn't find another test that would be writing to test.txt. I don't have an explanation for this. But I renamed the test.txt in test_stdlib_bitset_64 also and that seemed to fix the problem.

[certik]

I am going to merge this, as this seems to fix it. We will watch the master and see.

[jvdp1]

Thank you. Hopefully it is fixed indeed!