gopls/internal/cache: fix crash in snapshot.Analyze with patch versions

Fix the same crash as golang/go#66195, this time in Analyze: don't set
invalid Go versions on the types.Config.

The largest part of this change was writing a realistic test, since the
lack of a test for golang/go#66195 caused us to miss this additional
location. It was possible to create a test that worked, by using a flag
to select a go1.21 binary location.

For this test, it was required to move a couple additional integration
test preconditions into integration.Main (otherwise, the test would be
skipped due to the modified environment).

Fixes golang/go#66636

Change-Id: I24385474d4a6ebf6b7e9ae8f20948564bad3f55e
Reviewed-on: https://go-review.googlesource.com/c/tools/+/576135
Auto-Submit: Robert Findley <rfindley@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: findleyr

internal/testenv/testenv.go

@@ -45,7 +45,10 @@ var checkGoBuild struct {
 	err  error
 }
 
-func hasTool(tool string) error {
+// HasTool reports an error if the required tool is not available in PATH.
+//
+// For certain tools, it checks that the tool executable is correct.
+func HasTool(tool string) error {
 	if tool == "cgo" {
 		enabled, err := cgoEnabled(false)
 		if err != nil {
@@ -198,7 +201,7 @@ func allowMissingTool(tool string) bool {
 // NeedsTool skips t if the named tool is not present in the path.
 // As a special case, "cgo" means "go" is present and can compile cgo programs.
 func NeedsTool(t testing.TB, tool string) {
-	err := hasTool(tool)
+	err := HasTool(tool)
 	if err == nil {
 		return
 	}