Since MR !1025 fixed the Windows build, allowing us to build a binary
distribution, we can now run the testsuite in that CI job.

This required fixing 'createFileLink': it should not try to create
symlinks on Windows (that requires admin priviledges, which Hadrian can't
assume). We now instead fall back to copying.

This patch also removes some duplicated logic for iserv in the test rules,
where we handle our dependency on the iserv binaries in a special way.