This CL turns tests that call tool.Run (which includes
only a part of the initialization process and is not
a reliable simulation of the gopls executable)
into true CLI integration tests.

These missing tests would have caught regressions in recent work
that was rolled back before the release. (See CL 793600.)

Also, use correct URL in help message.

For golang/go#80133

Change-Id: I985b48e7b44d74e4d481172a2cbe5e9b5804984b
Reviewed-on: https://go-review.googlesource.com/c/tools/+/794300
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alex Putman <aputman@golang.org>