Refs #17513

It is enabled by default and disabled for builders.

For cases where file logging is enabled then the framework simply uses the
specified path as is or defaults to the appdata directory on all
platforms. This has the advantage of moving it outside if the install
directory.

Running unit tests in parallel causes all sorts of race condition issues.

Previously the index type was size_t, i.e., also 64 bit, but actually
contained the spectrum number, which was 32 bit, so casting to an int
did not cause trouble. Now we are using the this pointer as index, and
the values exceed 32 bit. This might explain the failing unit tests on
OSX (clang).