Add test for --json support for forget command

This adds a test of the json output of the forget command, by running it
once, asking it to keep one snapshot, and verifying that the output has
the right number of snapshots listed in the Keep and Remove fields of
the result.

cmd/restic/integration_test.go

@@ -219,6 +219,35 @@ func testRunForget(t testing.TB, gopts GlobalOptions, args ...string) {
 	rtest.OK(t, runForget(opts, gopts, args))
 }
 
+func testRunForgetJSON(t testing.TB, gopts GlobalOptions, args ...string) {
+	buf := bytes.NewBuffer(nil)
+	oldJSON := gopts.JSON
+	gopts.stdout = buf
+	gopts.JSON = true
+	defer func() {
+		gopts.stdout = os.Stdout
+		gopts.JSON = oldJSON
+	}()
+
+	opts := ForgetOptions{
+		DryRun: true,
+		Last:   1,
+	}
+
+	rtest.OK(t, runForget(opts, gopts, args))
+
+	var forgets []*ForgetGroup
+	rtest.OK(t, json.Unmarshal(buf.Bytes(), &forgets))
+
+	rtest.Assert(t, len(forgets) == 1,
+		"Expected 1 snapshot group, got %v", len(forgets))
+	rtest.Assert(t, len(forgets[0].Keep) == 1,
+		"Expected 1 snapshot to be kept, got %v", len(forgets[0].Keep))
+	rtest.Assert(t, len(forgets[0].Remove) == 2,
+		"Expected 2 snapshots to be removed, got %v", len(forgets[0].Remove))
+	return
+}
+
 func testRunPrune(t testing.TB, gopts GlobalOptions) {
 	rtest.OK(t, runPrune(gopts))
 }
@@ -1051,6 +1080,7 @@ func TestPrune(t *testing.T) {
 	rtest.Assert(t, len(snapshotIDs) == 3,
 		"expected 3 snapshot, got %v", snapshotIDs)
 
+	testRunForgetJSON(t, env.gopts)
 	testRunForget(t, env.gopts, firstSnapshot[0].String())
 	testRunPrune(t, env.gopts)
 	testRunCheck(t, env.gopts)