Add agent check-in interval to agent install tests

Omitting the agent check-in interval when patching the
`fleet-controller` config map would now lead to errors when setting up
agents with a check-in interval bearing the default value for a
duration, ie 0s.
That interval is now set with a hard-coded value, which is of no
importance for such tests, for the sake of not being zero.

e2e/multi-cluster/installation/agent_test.go

@@ -27,8 +27,9 @@ var _ = Describe("Fleet installation with TLS agent modes", func() {
 			"cattle-fleet-system",
 			"--type=merge",
 			"-p",
+			// Agent check-in interval cannot be 0. Any other value will do here.
 			fmt.Sprintf(
-				`{"data":{"config":"{\"apiServerURL\": \"https://google.com\", \"apiServerCA\": \"\", \"agentTLSMode\": \"%s\"}"}}`,
+				`{"data":{"config":"{\"apiServerURL\": \"https://google.com\", \"apiServerCA\": \"\", \"agentTLSMode\": \"%s\", \"agentCheckinInterval\": \"1m\"}"}}`,
 				agentMode,
 			),
 		)