internal/upload: split TestDates and remove setup

We can use a bogus count files without affecting the test process's
default count file or telemetry directory, by utilizing the new
internal/regtest.RunProg. We no longer need to interact with the
process-wide counter.Open for testing. Let's split this large
TestDates into multiple tests.

Each test and subtest interacts with a separate telemetry directory,
so we don't need to worry about the files created or left by other
test and subtest runs.

Also, replace the testServer with httptest test server. That handles
the server goroutine internally, and helps simplify the code flow.

And, finally, we can remove the `setup` function.

Change-Id: Id0a3057a702d4e3b93c60dfffd92fe402d817744
Reviewed-on: https://go-review.googlesource.com/c/telemetry/+/538297
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Robert Findley <[email protected]>