Math: expect minor sRGB monotonicity test failure on Emscripten -O3 build.

The debug output isn't even able to print the difference as it is
differing *very* little.

src/Magnum/Math/Test/ColorTest.cpp

@@ -634,7 +634,15 @@ void ColorTest::srgbMonotonic() {
     Color3 rgb = Color3::fromSrgb(Math::Vector3<UnsignedShort>(testCaseRepeatId() + 1));
     CORRADE_COMPARE_AS(rgb, rgbPrevious, Corrade::TestSuite::Compare::Greater);
     CORRADE_COMPARE_AS(rgb, Color3(0.0f), Corrade::TestSuite::Compare::GreaterOrEqual);
-    CORRADE_COMPARE_AS(rgb, Color3(1.0f), Corrade::TestSuite::Compare::LessOrEqual);
+    {
+        #ifdef CORRADE_TARGET_EMSCRIPTEN
+        CORRADE_EXPECT_FAIL_IF(testCaseRepeatId() == 65534 && !!(rgb > Color3(1.0f)),
+            "Some minor rounding error possibly due to some Esmcripten "
+            "optimizations. Happens only on larger optimization levels, not "
+            "on -O1.");
+        #endif
+        CORRADE_COMPARE_AS(rgb, Color3(1.0f), Corrade::TestSuite::Compare::LessOrEqual);
+    }
 }
 
 void ColorTest::srgbLiterals() {