Test that a branch is tried before its pull request (hq issue 686)

A builder's store tests first ran in the pull request's merge check, a review
round late. These fail on main: a try asks the repository's own check alone,
its outcome sets no status and registers nothing, an unknown ref is refused
before anything is asked, and the answer carries the failing lines.

internal/builder/check_test.go

@@ -590,3 +590,15 @@ func TestARepositoryInTwoLanguagesIsCheckedInBoth(t *testing.T) {
 		t.Errorf("a part in a toolchain the mesh does not hold answered %+v — never a pass", layer)
 	}
 }
+
+// A branch tried before its pull request may narrow what the repository's script runs (novox/hq issue 686); a
+// pull request's check is handed nothing, and runs whole.
+func TestOnlyATrysSelectionReachesTheScript(t *testing.T) {
+	if env := selection(CheckSpec{Number: 7}); len(env) != 0 {
+		t.Fatalf("a pull request's check was handed a selection: %v", env)
+	}
+	env := selection(CheckSpec{Tests: "^TestX$", StoreOnly: true})
+	if len(env) != 2 || env[0] != "MESH_CHECK_TESTS=^TestX$" || env[1] != "MESH_CHECK_STORE_ONLY=1" {
+		t.Fatalf("a try's selection: %v", env)
+	}
+}