First off, I'm confused - what premium service? There is no premium service available for spousal visa applications because the visa is processed in Sheffield and you have no choice but to mail it there from the US.
And you cannot apply in person in the UK unless you are already living in the UK on a different visa valid more than 6 months (or a fiancé visa which is valid 6 months) and are switching to FLR(M).
So, if you are in the UK as a visitor right now and you attempt to apply in person at a premium service centre, you will be turned away/your visa will be refused because you don't qualify to apply from inside the UK.
So if you have a premium appointment booked and you do not currently hold a fiancé, work or student visa allowing you to live in the UK, you need to cancel it and request a refund, then fly back to the US to apply for the spousal visa by mail.
To answer your question, no, the Life in the UK test is not required until you apply for ILR after 5 years in the UK on spousal/FLR(M) visas.
You can take the test at any time once you have your spousal visa (providing you have proof of your UK address by way of a utility bill or similar) as the pass certificate never expires but you don't need to do it until you are about to apply for ILR.
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