[clang-tidy] Show better diff when check_alphabetical_order.py fails

Author: vbvictor

clang-tools-extra/test/clang-tidy/infrastructure/alphabetical-order.test

@@ -1,5 +1,5 @@
 // RUN: %python %S/../../../clang-tidy/tool/check_alphabetical_order.py -o %t.clang-tidy-checks-list.rst
-// RUN: diff --strip-trailing-cr %t.clang-tidy-checks-list.rst %S/../../../docs/clang-tidy/checks/list.rst
+// RUN: diff --strip-trailing-cr %S/../../../docs/clang-tidy/checks/list.rst %t.clang-tidy-checks-list.rst
 
 // RUN: %python %S/../../../clang-tidy/tool/check_alphabetical_order.py -o %t.ReleaseNotes.rst
-// RUN: diff --strip-trailing-cr %t.ReleaseNotes.rst %S/../../../docs/ReleaseNotes.rst
+// RUN: diff --strip-trailing-cr %S/../../../docs/ReleaseNotes.rst %t.ReleaseNotes.rst