The existing tests use "C:/CMakeLists.txt", which is only absolute on
Windows. ToNormalizedPathOnDisk resolves relative paths against the
working directory, so give the tests a platform-appropriate absolute
path via kTestSourcePath.
Add three tests that set breakpoints using a different letter case
than the path passed to SourceFileLoaded or GetBreakpoints, covering
all three NormalizePath call sites. The tests create a real file on
disk so that ToNormalizedPathOnDisk can resolve the canonical case,
and skip gracefully on platforms where ToNormalizedPathOnDisk does
not correct on-disk case (case-sensitive filesystems, or CYGWIN which
is treated as POSIX).
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