gopls/internal/regtest: show line numbers if TestUseGoplsMod fails
gopls/internal/regtest: show line numbers if TestUseGoplsMod fails

I can't repro the failure now, but on a few earlier CLs this week, this
test failed. Getting the actual line number in the failure message and
failing early would still be helpful, so I think this CL is still worth
merging in case it happens again.

Change-Id: I5ef0490ed103bea03aed018b45c9d1f383decc70
Reviewed-on: https://go-review.googlesource.com/c/tools/+/280533
Run-TryBot: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>
gopls-CI: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Suzy Mueller <a1fd7fee333bfb1457f0c89ab258f70a76be3c5d@golang.org>
Trust: Suzy Mueller <a1fd7fee333bfb1457f0c89ab258f70a76be3c5d@golang.org>
Trust: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools