Fix test_files-clang failure with Clang 13

The test fails on my system and on KDevelop CI like this:
QDEBUG : TestFiles::testFiles(basicdeclsandcontexts.cpp) "identifier" FAILED: Declaration's identifier ("(unnamed struct at /home/jenkins/workspace/KDevelop/kdevelop/kf5-qt5 SUSEQt5.15/plugins/clang/tests/files/basicdeclsandcontexts.cpp:97:1)") doesn't match test data (""). (Declaration on line 97 in /home/jenkins/workspace/KDevelop/kdevelop/kf5-qt5 SUSEQt5.15/plugins/clang/tests/files/basicdeclsandcontexts.cpp)
QDEBUG : TestFiles::testFiles(basicdeclsandcontexts.cpp) "identifier" FAILED: Declaration's identifier ("(unnamed union at /home/jenkins/workspace/KDevelop/kdevelop/kf5-qt5 SUSEQt5.15/plugins/clang/tests/files/basicdeclsandcontexts.cpp:100:1)") doesn't match test data (""). (Declaration on line 100 in /home/jenkins/workspace/KDevelop/kdevelop/kf5-qt5 SUSEQt5.15/plugins/clang/tests/files/basicdeclsandcontexts.cpp)
FAIL!  : TestFiles::testFiles(basicdeclsandcontexts.cpp) 'validator.testsPassed()' returned FALSE. ()
   Loc: [/home/jenkins/workspace/KDevelop/kdevelop/kf5-qt5 SUSEQt5.15/plugins/clang/tests/test_files.cpp(116)]

The test fix is based on adding testDataAdjuster callback to
DeclarationValidator. It should be possible to reuse this new interface
to solve future test_files* failures.

I haven't verified that the test passes with older Clang versions. But
made sure that the test data is adjusted to the values from the parent
commit when Clang version is older than 13. So it should pass.