Fix TestAccSqlUser_postgresIAM by waiting for user to be created

[modular-magician]

fixes hashicorp/terraform-provider-google#14221

After the SQL instance is created, it appears that the cloudsqliamuser role is not immediately available, so the user creation fails. To fix this I introduced a delay before user creation.

I tried a 1 second delay (still saw the error) and a 10 second delay (error was resolved), so to be safe I went with a 30 second delay.

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Derived from GoogleCloudPlatform/magic-modules#7638