tstime/mono: fix Time.Unmarshal (#8480)

Calling both mono.Now() and time.Now() is slow and
leads to unnecessary precision errors.
Instead, directly compute mono.Time relative to baseMono and baseWall.
This is the opposite calculation as mono.Time.WallTime.

Updates tailscale/corp#8427

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

tstime/mono/mono_test.go

@@ -33,6 +33,21 @@ func TestUnmarshalZero(t *testing.T) {
 	}
 }
 
+func TestJSONRoundtrip(t *testing.T) {
+	want := Now()
+	b, err := want.MarshalJSON()
+	if err != nil {
+		t.Errorf("MarshalJSON error: %v", err)
+	}
+	var got Time
+	if err := got.UnmarshalJSON(b); err != nil {
+		t.Errorf("UnmarshalJSON error: %v", err)
+	}
+	if got != want {
+		t.Errorf("got %v, want %v", got, want)
+	}
+}
+
 func BenchmarkMonoNow(b *testing.B) {
 	b.ReportAllocs()
 	for i := 0; i < b.N; i++ {