gopls/internal/regtest/modfile: set an explicit go version in the Tes…

…tUnknownRevision modules

For some reason the "bad" case of this test fails when the main module
is lazy. Since the go.mod file lacks a 'go' version directive, as of
CL 315210 it is upgraded to a lazy module the first time it is edited
by an invocation of cmd/go.

I don't know why this case doesn't exhibit the expected failure mode
when lazy loading is enabled — I guess it's because we attribute the
checksum error to the individual package that needs it instead of the
module as a whole? — but I can't follow this test well enough to
figure out whether there is actually a real problem for lazy modules
here. If there is, we can handle that by adding a new, separate test.

For golang/go#36460

Change-Id: I0949e1f9f5cb1b6c884706e50a9694232308387b
Reviewed-on: https://go-review.googlesource.com/c/tools/+/315152
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>

gopls/internal/regtest/modfile/modfile_test.go

@@ -519,6 +519,8 @@ func TestUnknownRevision(t *testing.T) {
 -- a/go.mod --
 module mod.com
 
+go 1.16
+
 require (
 	example.com v1.2.2
 )
@@ -567,6 +569,8 @@ func main() {
 -- a/go.mod --
 module mod.com
 
+go 1.16
+
 require (
 	example.com v1.2.3
 )