- Fix a bug in the logic to suppress a context cancellation error
  computing the shared import graph.
- Remove ErrIsDefinition. It was introduced for testing in CL 196560,
  and is no longer needed.
- Update golang.SignatureHelp to return (nil, nil) in the case where
  SignatureHelp is simply not available.
- Suppress the mod tidy diagnostic error when GOPROXY=off. It is too
  noisy, and golang/go#56395 tracks the fix for the root cause.
- Suppress noisy errors about semantic tokens. Researching the history
  behind this error, it seems it was only added to force the
  invalidation of semantic tokens client-side. The same can be achieved
  by returning a non-nil, empty result.
- Enforce that marker tests encounter no error logs, with an -errors_ok
  flag to allow them in select cases.
- Update the -min_go version of some marker tests that use a go.mod file
  with a too-new go version. This was an error log.

For golang/go#66746

Change-Id: I21fe274e320cf5fa7d6b85d7402330fb647dbd29
Reviewed-on: https://go-review.googlesource.com/c/tools/+/577655
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>