clang-tidy: don't use an absolute path in a test

`run_clang_tidy` takes a regular expression to match against
compile_commands.json entries. If we pass "%t/test.cpp" and "%t" expands
to anything that includes chars that a regex treats specially, like '+',
this test starts failing.

clang-tools-extra/test/clang-tidy/infrastructure/run-clang-tidy.cpp

@@ -9,7 +9,7 @@
 // RUN: echo "    value:           '0'" >> %t/.clang-tidy
 // RUN: cp "%s" "%t/test.cpp"
 // RUN: cd "%t"
-// RUN: not %run_clang_tidy "%t/test.cpp"
+// RUN: not %run_clang_tidy "test.cpp"
 
 int main()
 {