fix problem on windows with newly rewritten prop_relPathDirToFileAbs_basics

fix problem on windows with newly rewritten prop_relPathDirToFileAbs_basics

Seems that dropDrive on windows only drops eg c:/ but not a leading /
while on linux, it does drop a leading / (which is what it considers
to be equivilant to a drive letter. I had been relying on it to drop
both. So need to drop leading directory separators.

Also, if the quickcheck generated input is eg "c:c:c:c:foo",
dropDrive will only drop the first one, leaving a path that's
still not relative. So instead of using dropDrive, just remove the
colons from the path.