clang-tools-extra/test/clang-tidy/line-filter.cpp: Tweak line-filter.

On win32, %s is expanded to X:\path\to\test\line-filter.cpp. It was incompatible to yaml.
Although "%/s" could be available in Lit, ClangTidyDiagnosticConsumer::passesLineFilter() is unaware of comparision between '/' and '\\'.