The testsuite can run only on Linux, because it requires `xvfb-run`,
which is not available on non-Linux platforms; however, it should not be
limited just to `x86_64-linux`.
The conditional setting of `checkPhase` was introduced in #6135, because
at that time the `checkPhase` script referred to `${xdummy}`, and that
kind of reference would break evaluation if the specified package was
not available; the current version of the `checkPhase` script does not
refer to any package paths directly, therefore the condition is no
longer needed and can be removed.
43e1972a68