[lldb/test] Fix TestDSYMSourcePathRemapping in the presence of symlnks

My main work directory is on a separate partition, but I usually access
it through a symlink in my home directory. When running the tests,
either Clang or make resolves the symlink, and the real path of the
test directory ends up in the debug information.

This confuses this test as LLDB is trying to remap the real path, but
the remapping description uses the path with the symlink in
it. Calling realpath on the source path when constructing the
remapping description fixes it.