Add custom-media syntax tests

Author: deathaxe

tests/syntax_test_less.less

@@ -318,6 +318,13 @@ a, .link { color: @@color; }
 //            ^^ entity.other.custom-media.css - meta.interpolation
 //              ^^^^^^^^^ entity.other.custom-media.css meta.interpolation.less
 //                       ^^^^^^ entity.other.custom-media.css - meta.interpolation
+//                              ^ punctuation.section.group.begin.css
+//                               ^^^^^ support.type.property-name.css
+//                                    ^ punctuation.separator.key-value.css
+//                                      ^ meta.number.integer.decimal.css constant.numeric.value.css
+//                                       ^^ meta.number.integer.decimal.css constant.numeric.suffix.css
+//                                         ^ punctuation.section.group.end.css
+//                                          ^ punctuation.terminator.rule.css
 
 @import "@{themes}/tidal-wave.less";
 //^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ meta.at-rule.import.css