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Break up test cases into 5 different groups. (Purely cosmetic.)

Rather than add the new test functions I had considered putting groups of tests in anonymous blocks ({}s) and preceding with a comment naming the group.
Add "macro expansion" test.

Libclang::get_offset() now uses clang_getFileLocation(), although I could have probably left it using clang_getExpansionLocation() as I didn't detect any functional differences (nor with using clang_getSpellingLocation).
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