Work around GCC bug in GCC 4.8

Error:
/home/foo/kdesrc/kdevelop/languages/clang/tests/test_assistants.cpp:62:53:
error: unterminated raw string
     QLoggingCategory::setFilterRules(QStringLiteral(R"(
                                                     ^
See:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55971