GL: make spurious test failures with queries more bearable.

The XFAIL can be disabled globally, if needed.

src/Magnum/GL/Test/PrimitiveQueryGLTest.cpp

@@ -169,7 +169,10 @@ void PrimitiveQueryGLTest::primitivesGenerated() {
     const bool availableAfter = q.resultAvailable();
 
     MAGNUM_VERIFY_NO_GL_ERROR();
+    {
+        CORRADE_EXPECT_FAIL_IF(availableBefore, "GPU faster than light?");
         CORRADE_VERIFY(!availableBefore);
+    }
     CORRADE_VERIFY(availableAfter);
     CORRADE_COMPARE(count, 3);
 }

src/Magnum/GL/Test/SampleQueryGLTest.cpp

@@ -194,7 +194,10 @@ void SampleQueryGLTest::querySamplesPassed() {
     const bool availableAfter = q.resultAvailable();
 
     MAGNUM_VERIFY_NO_GL_ERROR();
+    {
+        CORRADE_EXPECT_FAIL_IF(availableBefore, "GPU faster than light?");
         CORRADE_VERIFY(!availableBefore);
+    }
     CORRADE_VERIFY(availableAfter);
     #ifndef MAGNUM_TARGET_GLES
     CORRADE_COMPARE(count, 32*32);