Clean up use of FHIRRequestContext in unit tests

[lmsurpre]

Is your feature request related to a problem? Please describe.
We use a threadlocal to store important request context like the tenantId and dataStoreId.

In our tests, we often need to set the request context since its outside of the server and so the requestFilter doesn't set it for us.

The pattern is like this:

FHIRRequestContext context = FHIRRequestContext.get();
// do stuff with context
FHIRRequestContext.set(context);

or, equivalently

FHIRRequestContext context = FHIRRequestContext.get();
FHIRRequestContext.set(context);

// do stuff with context

However, because the object is already associated with the local thread when we call get(), there's no need to call set with it.

Describe the solution you'd like
Remove the unnecessary calls to FHIRRequestContext.set(context);

Describe alternatives you've considered
Finally move to CDI and have the request context injected all over

Acceptance Criteria

1. GIVEN [a precondition]
   AND [another precondition]
   WHEN [test step]
   AND [test step]
   THEN [verification step]
   AND [verification step]

Additional context
Now that we don't allow any null values in the ThreadLocal (#3282), we can remove the null check any tests that are guarding against a null FHIRRequestContext.
Hint: look for if (context == null) {

[punktilious]

On the other hand, it concerns me that a test is relying on a particular state but isn't doing anything to establish that state (or check that it's correct). Also, we may have some tests which are looking for null in order to set up the context, so as pointed out, it will be good to look for if (context == null) and examine the local logic.

[lmsurpre]

these changes are pretty isolated to test code...will be tested as part of general regression testing before release