Fix flaky operator debug test

[tgross]

Fixes #12488

We introduced a pprof-interval argument to operator debug in #11938, and unfortunately this has resulted in a lot of test flakes. The actual command in use is mostly fine (although I've fixed some quirks here), so what's really happened is that the change has revealed some existing issues in the tests. Best reviewed commit-by-commit but a summary of the changes is below. (No changelog entry because this has only shipped for 1.3-beta.1)

• Make first pprof collection synchronous to preserve the existing
  behavior for the common case where the pprof interval matches the
  duration.

• Clamp operator debug pprof timing to that of the command. The
  pprof-duration should be no more than duration and the
  pprof-interval should be no more than pprof-duration. Clamp the
  values rather than throwing errors, which could change the commands
  that existing users might already have in debugging scripts

• Testing: remove test parallelism

  The operator debug tests that stand up servers can't be run in
  parallel, because we don't have a way of canceling the API calls for
  pprof. The agent will still be running the last pprof when we exit,
  and that breaks the next test that talks to that same agent.
  (Because you can only run one pprof at a time on any process!)

  We could split off each subtest into its own server, but this test
  suite is already very slow. In future work we should fix this "for
  real" by making the API call cancelable.

• Testing: assert against unexpected errors in operator debug tests.

  If we assert there are no unexpected error outputs, it's easier for
  the developer to debug when something is going wrong with the tests
  because the error output will be presented as a failing test, rather
  than just a failing exit code check. Or worse, no failing exit code
  check!

  This also forces us to be explicit about which tests will return 0
  exit codes but still emit (presumably ignorable) error outputs.

Additional minor bug fixes (mostly in tests) and test refactorings:

• Fix text alignment on pprof Duration in operator debug output

• Remove "done" channel from operator debug event stream test. The
  goroutine we're blocking for here already tells us it's done by
  sending a value, so block on that instead of an extraneous channel

• Event stream test timer should start at current time, not zero

• Remove noise from operator debug test log output. The t.Logf
  calls already are picked out from the rest of the test output by
  being prefixed with the filename.

• Remove explicit pprof args so we use the defaults clamped from
  duration/interval

[shoenig]

LGTM!

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