Address external_issuer_test.go flakiness (#4018)

From time to time we get this CI error when testing the external issuer
mechanism:
```
Test script: [external_issuer_test.go] Params:
[--linkerd-namespace=l5d-integration-external-issuer
--external-issuer=true]
--- FAIL: TestExternalIssuer (33.61s)
    external_issuer_test.go:89: Received error while ensuring test app
    works (before cert rotation): Error stripping header and trailing
    newline; full output:
    FAIL
```
https://github.com/alpeb/linkerd2/runs/428273855?check_suite_focus=true#step:6:526

This is caused by the "backend" pod not receiving traffic from
"slow-cooker" in a timely manner.
After those pods are deployed we're only checking that "backend" is
ready, but not "slow-cooker", so this change adds that check.

I'm also removing the `TestHelper.CheckDeployment` call because it's
redundant, since the preceeding `TestHelper.CheckPods` is already checking
that the deployment has all the specified replicas ready.

test/externalissuer/external_issuer_test.go

@@ -52,13 +52,12 @@ func verifyInstallApp(t *testing.T) {
 		t.Fatalf("kubectl apply command failed\n%s", out)
 	}
 
-	// wait for deployment to start
 	if err := TestHelper.CheckPods(prefixedNs, TestAppBackendDeploymentName, 1); err != nil {
 		t.Error(err)
 	}
 
-	if err := TestHelper.CheckDeployment(prefixedNs, TestAppBackendDeploymentName, 1); err != nil {
-		t.Error(fmt.Errorf("Error validating deployment [%s]:\n%s", TestAppBackendDeploymentName, err))
+	if err := TestHelper.CheckPods(prefixedNs, "slow-cooker", 1); err != nil {
+		t.Error(err)
 	}
 }