In prior commits in this PR we added several new test functions to our
"lfsapi" package to verify the behaviour of the DoWithAuth() method
of that package's Client structure.

These new functions include the TestDoWithAuthRetryLimitExceeded()
function and the TestDoWithAuthMultistageRetryLimitExceeded() function,
both of which check that the DoWithAuth() method now limits the number
of HTTP requests it will make if each request receives a 401 Unauthorized
status code in its response.

Both test functions follow a similar design as the existing
TestDoWithAuthApprove() function.  That older test function ends with a
check of the access mode which has been cached for the endpoint URL
associated with the HTTP requests made by the DoWithAuth() method, as
we expect that after the first 401 status code response is received,
this mode will be set to the BasicAccess mode from our "creds" package
to reflect that HTTP Basic Authentication should be used with all
subsequent requests.

Although our new TestDoWithAuthRetryLimitExceeded() function also ends
with a check of the cached access mode, this is not yet the case for
the TestDoWithAuthMultistageRetryLimitExceeded() function, so we add
that check now.

We also take the opportunity to simplify all of the access mode assertion
checks we make, which at present explicitly pass structure pointers
as the receiver parameters of the Mode() method of the Access structure
from our "creds" package.  While this method is defined with a pointer
receiver, it is not necessary to explicitly generate these pointers
because Go automatically does so as a convenience feature of the language:

  https://go.dev/tour/methods/6

The use of explicit pointer receivers for the Mode() method calls in our
test functions dates from commit 6d29072
in PR git-lfs#3941, when the Access structure was first moved into our "creds"
package.