Text: actually make use of the test instance parameters.

Not sure what was I doing in 9d1f2f9bc9.

src/Magnum/Text/Test/DistanceFieldGlyphCacheGLTest.cpp

@@ -109,6 +109,8 @@ const struct {
     bool glPixelFormat;
 } SetProcessedImageData[]{
     {"", false, false},
+    {"using deprecated API", true, false},
+    {"using deprecated API and GL-specific pixel format", true, true},
 };
 #endif