Make TabStatsTracker unit tests less fragile

When TabStatsTracker is enabled on Android, a lot of these tests fail
because they expect a number of objects to exactly match a histogram
sample. But the histogram is exponentially bucketed and HistogramTester
only checks for the start of a bucket. On desktop platforms the number
of objects happens to fall at the start of the bucket, but Android
creates one more platform object, so the object count falls inside the
bucket but the histogram sample is the start of the bucket.

This adds ExpectBucketedSample methods to the unit test that check
whether a value is between the min and max of a bucket.

Also:

* Removes observers at the end of every test, in case a platform sends
  notifications during teardown.
* Converts CHECK's to ASSERTs that fail the test without crashing.
* Removes code that sets up TabGroups but doesn't actually test
  anything in TabStatsTracker.

Bug: 412634171
Change-Id: I1a76c06ef6de03c7d4269aa425575696cb669f11
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6486167
Commit-Queue: Joe Mason <joenotcharles@google.com>
Auto-Submit: Joe Mason <joenotcharles@google.com>
Reviewed-by: Olivier Li <olivierli@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1452817}