Audio: some more converage improvements.

package/archlinux/PKGBUILD-coverage

@@ -93,6 +93,7 @@ check() {
     done
 
     ./Debug/bin/magnum-al-info > /dev/null
+    ./Debug/bin/magnum-al-info --extension-strings > /dev/null
     ./Debug/bin/magnum-gl-info --limits > /dev/null
     # Not calling vk-info because currently it doesn't do anything that
     # wouldn't be covered elsewhere (but it causes false positives in coverage)

src/Magnum/Audio/Test/PlayableALTest.cpp

@@ -79,6 +79,7 @@ void PlayableALTest::group() {
     CORRADE_COMPARE(playable.source().gain(), 0.5f);
 
     playable.setGain(0.5f);
+    CORRADE_COMPARE(playable.gain(), 0.5f);
     CORRADE_COMPARE(playable.source().gain(), 0.25f);
 
     group.play();