Pushrule tweaks, make pattern non-optional on EventMatchCondition (#2918)

This should fix https://github.com/matrix-org/dendrite/issues/2882
(Tested with FluffyChat 1.7.1)
Also adds tests that the predefined push rules (as per the spec) is what
we have in Dendrite.

internal/pushrules/default_content.go

@@ -15,13 +15,7 @@ func mRuleContainsUserNameDefinition(localpart string) *Rule {
 		RuleID:  MRuleContainsUserName,
 		Default: true,
 		Enabled: true,
-		Pattern: localpart,
-		Conditions: []*Condition{
-			{
-				Kind: EventMatchCondition,
-				Key:  "content.body",
-			},
-		},
+		Pattern: &localpart,
 		Actions: []*Action{
 			{Kind: NotifyAction},
 			{
@@ -32,7 +26,6 @@ func mRuleContainsUserNameDefinition(localpart string) *Rule {
 			{
 				Kind:  SetTweakAction,
 				Tweak: HighlightTweak,
-				Value: true,
 			},
 		},
 	}