check: disable exhaustive checks in test files

It's not quite clear how to make it work. In cases where a package both
defines tests and a sum type, you wind up with multiple distinct
definitions of types that do not satisfy types.Identical, which in turn
results in inexhaustive errors. It's not quite clear how to fix this.

main.go

@@ -53,8 +53,18 @@ func run(pkgs []*packages.Package) []error {
 
 func tycheckAll(args []string) ([]*packages.Package, error) {
 	conf := &packages.Config{
-		Mode:  packages.LoadSyntax,
-		Tests: true,
+		Mode: packages.LoadSyntax,
+		// Unfortunately, it appears including the test packages in
+		// this lint makes it difficult to do exhaustiveness checking.
+		// Namely, it appears that compiling the test version of a
+		// package introduces distinct types from the normal version
+		// of the package, which will always result in inexhaustive
+		// errors whenever a package both defines a sum type and has
+		// tests. (Specifically, using `package name`. Using `package
+		// name_test` is OK.)
+		//
+		// It's not clear what the best way to fix this is. :-(
+		Tests: false,
 	}
 	pkgs, err := packages.Load(conf, args...)
 	if err != nil {