spanchecks: Support testing without tracing (#4682)

Tests cannot be ran without configuring tracing. Split from #4678.

Does not nag about the span checks when there is no subscriber
configured, because then the spans will have no links and nothing can be
checked. Sadly the `SpanTrace::status()` cannot be used for this.
`tracing` is always configured in regress testing (running with
`pageserver` binary), which should be enough.

Additionally cleans up the test code in span checks to be in the test
code. Fixes a `#[should_panic]` test which was flaky before these
changes, but the `#[should_panic]` hid the flakyness.

Rationale for need: Unit tests might not be testing only the public or
`feature="testing"` APIs which are only testable within `regress` tests
so not all spans might be configured.

Author: koivunej