chore: remove parallel running in integration tests (#8904)

# Which Problems Are Solved

Integration tests are flaky due to eventual consistency.

# How the Problems Are Solved

Remove t.Parallel so that less concurrent requests on multiple instance
happen. This allows the projections to catch up more easily.

# Additional Changes

- none

# Additional Context

- none

internal/api/grpc/system/integration_test/instance_test.go

@@ -15,8 +15,6 @@ import (
 )
 
 func TestServer_ListInstances(t *testing.T) {
-	t.Parallel()
-
 	isoInstance := integration.NewInstance(CTX)
 
 	tests := []struct {

internal/api/grpc/system/integration_test/limits_auditlogretention_test.go

@@ -22,8 +22,6 @@ import (
 )
 
 func TestServer_Limits_AuditLogRetention(t *testing.T) {
-	t.Parallel()
-
 	isoInstance := integration.NewInstance(CTX)
 	iamOwnerCtx := isoInstance.WithAuthorization(CTX, integration.UserTypeIAMOwner)
 	userID, projectID, appID, projectGrantID := seedObjects(iamOwnerCtx, t, isoInstance.Client)

internal/api/grpc/system/integration_test/limits_block_test.go

@@ -26,8 +26,6 @@ import (
 )
 
 func TestServer_Limits_Block(t *testing.T) {
-	t.Parallel()
-
 	isoInstance := integration.NewInstance(CTX)
 	iamOwnerCtx := isoInstance.WithAuthorization(CTX, integration.UserTypeIAMOwner)
 	tests := []*test{