Use POSIX-style paths even on Windows.
It makes all reported paths look less\jumbled/with\mixed/slashes.

git-svn-id: http://include-what-you-use.googlecode.com/svn/trunk@532 812d1f90-f944-d88a-411a-122bdc93e1ea
Inline TEST_ROOTDIR_CXX and TEST_ROOTDIR_C.
They were only used in one place anyway.

git-svn-id: http://include-what-you-use.googlecode.com/svn/trunk@533 812d1f90-f944-d88a-411a-122bdc93e1ea
Add PosixPath helper function.
Paths were previously normalized in two places, in different ways.

git-svn-id: http://include-what-you-use.googlecode.com/svn/trunk@534 812d1f90-f944-d88a-411a-122bdc93e1ea