CTest: Teach --show-only=json-v1 to filter out not-available tests

Avoid exposing the internal special value that we use to track tests not
available in the tested configuration.  This also prevents clients from
having to do the filtering themselves.

Source/CTest/cmCTestMultiProcessHandler.cxx

@@ -1039,6 +1039,11 @@ void cmCTestMultiProcessHandler::PrintOutputAsJson()
     testRun.SetTestProperties(&p);
     testRun.ComputeArguments();
 
+    // Skip tests not available in this configuration.
+    if (p.Args.size() >= 2 && p.Args[1] == "NOT_AVAILABLE") {
+      continue;
+    }
+
     Json::Value testInfo = DumpCTestInfo(testRun, p, backtraceGraph);
     tests.append(testInfo);
   }

Tests/RunCMake/CTestCommandLine/RunCMakeTest.cmake

@@ -201,6 +201,7 @@ function(run_ShowOnly)
   file(WRITE "${RunCMake_TEST_BINARY_DIR}/CTestTestfile.cmake" "
     add_test(ShowOnly \"${CMAKE_COMMAND}\" -E echo)
     set_tests_properties(ShowOnly PROPERTIES WILL_FAIL true _BACKTRACE_TRIPLES \"file1;1;add_test;file0;;\")
+    add_test(ShowOnlyNotAvailable NOT_AVAILABLE)
 ")
   run_cmake_command(show-only_json-v1 ${CMAKE_CTEST_COMMAND} --show-only=json-v1)
 endfunction()