XFile::Append unit test failure

[DanRStevens]

Unit tests for XFile::Append are failing on Ubuntu 20.04, with both GCC and Clang:

[ RUN ] XFile.Append
test/XFile.test.cpp:39: Failure
Expected equality of these values:
"a/b/"
XFile::Append("a/", "b/")
Which is: "a/b/."
test/XFile.test.cpp:40: Failure
Expected equality of these values:
"/a/b/"
XFile::Append("/a/", "b/")
Which is: "/a/b/."
test/XFile.test.cpp:42: Failure
Expected equality of these values:
"C:a/b/"
XFile::Append("C:a/", "b/")
Which is: "C:a/b/."
test/XFile.test.cpp:43: Failure
Expected equality of these values:
"C:/a/b/"
XFile::Append("C:/a/", "b/")
Which is: "C:/a/b/."
[ FAILED ] XFile.Append (0 ms)

It seems that paths representing folders are marked with a . for the final component, rather than ending with /. This looks odd and may be unexpected. It's also inconsistent with behaviour on Windows, where paths do end with a /. Perhaps we should look at special casing this to ensure paths representing a folder end with /.

As a potential workaround for unit test failures, we can update the unit tests to use paths with filenames. That will avoid triggering the inconsistent behavior. It's not really a solution though, since it ignores the problem case, leaving it untested. Effectively we can hold off on defining how the problem case should be resolved.