"how do i test the switch?"

Here's a question: Does the wiper work on the "intermittent" setting?
(probably not, or you would have stated so)
If it does, then motor OK, problem elsewhere.
If not working on intermittent or regular "wipe", then
you still need to trace down the problem component.

Switch testing:
Since it's not the type of switch that springs back after you push it (like the washer button), the wiper switch would pass current as long as it's switched over to the "run the wipers" position.

Of course you'll need to figure out which 2 terminals are involved.
You can use a continuity tester or a multitester (AKA Volt-Ohm meter).

You may or may not need to remove switch from car.
Removing the lower and possibly upper plastic cover shells may get you enough access to the switch terminals.
For the turn signal switch, two good-size screws hold it on the steering column.
Probably the same for the wiper switch.
In any case, disconnect the switch from the rest of the wire harness - I'd guess there's a multi-pin connector for that - so you can test the switch itself directly.

Switch to "intermittent", then find 2 terminals on switch that show current passing between them.

That is, if using a continuity tester, touch the 2 terminals simultaneously, and it will light up or beep. If using an ohm-meter, it will show 0 ohms.

Test the other combinations of terminals to verify that it's only that one combination that passes current for that switch position.

Switch to "wipe". Find 2 terminals that pass current - same procedure.
Would likely be one of the 2 terminals that worked for the intermittent setting, plus one different one. But not guaranteed.

Anyway, if you can find 2 terminals that pass current through the switch at the appropriate switch settings, then the switch looks good.

Being super-thorough would involve learning exactly which color wires run what, etc. You may or may not ever find that information, but I don't think it's important. If one combination of wires works for "wipe" and another works for "intermittent", that should be good.

Also -
Lots of old relays out there have bad solder joints by now. If you replaced yours with another old one, it could be bad.

Oh, yeah - check out the relay setup.
Have a look at www.fcpgroton.com
Go to Volvo 240 parts - then "Relay's and Switches"
Shows the relays.
They show relay one for front wipers, one for rear (wagons), only for the "interval" function.
Apparently none just for the wipers or washers, but I dunno for certain.
It could be that regular "wipe" setting also requires that the relay has to be good, if the current takes that route to the motor.