test: Revamp which linux paths are redirected

test: Revamp which linux paths are redirected

Adds FALLBACK_DATA_DIRS, FALLBACK_CONFIG_DIRS, SYSCONFDIR, and EXTRASYSCONFDIR
to the list of redirected paths. They were previously 'covered' by manual entries.
This commit makes them responsive to changes in the build system.

This commit also makes sure that the path used for redirection is one searched by
the loader by using the SYSCONFIG variable. Previously it would use /usr/local/etc/
which was only correct on systems where CMAKE_INSTALL_PREFIX was set to /usr/local.