QPlainTestLogger: code tidies

Remove a couple of unnecessary string literals that only coped with an
impossible situation (default return after a switch that cover all the
enumerators).
Add a Q_UNREACHABLE() for good measure.

Change-Id: I8065218554edf98408d9c0823b8c301932681c59
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/testlib/qplaintestlogger.cpp

@@ -96,7 +96,8 @@ namespace QTest {
         case QAbstractTestLogger::BlacklistedXFail:
             return "BXFAIL ";
         }
-        return "??????";
+        Q_UNREACHABLE();
+        return nullptr;
     }
 
     static const char *benchmarkResult2String()
@@ -122,7 +123,8 @@ namespace QTest {
         case QAbstractTestLogger::Warn:
             return "WARNING";
         }
-        return "??????";
+        Q_UNREACHABLE();
+        return nullptr;
     }
 
     template <typename T>