[run.d] print failed command for failed unit tests (#20620)

compiler/test/run.d

@@ -199,7 +199,7 @@ Options:
             {
                 const string name = target.filename
                             ? target.normalizedTestName
-                            : "`unit` tests";
+                            : "`unit` tests: " ~ (cast(string)unitTestRunnerCommand) ~ join(target.args, " ");
 
                 writeln(">>> TARGET FAILED: ", name);
                 synchronized failedTargets ~= name;