Added GN target for isac_test.

.gn

Index: .gn
diff --git a/.gn b/.gn
index 6bbe648be649419fd1f955fa8d472a575465ff9a..a5f070a97c88624fd65211e23333fbcfe1cc2670 100644
--- a/.gn
+++ b/.gn
@@ -20,12 +20,13 @@ secondary_source = "//build/secondary/"
 # "gn check" or "gn gen --check".
 # TODO(kjellander): Keep adding paths to this list as work in webrtc:5589 is done.
 check_targets = [
-  "//webrtc/modules/audio_device/*",
-  "//webrtc/voice_engine:level_indicator",
-  "//webrtc/modules/audio_coding:isac_fix_test",
-  "//webrtc/modules/audio_mixer:audio_conference_mixer",
   "//webrtc/modules/audio_coding:g711_test",
   "//webrtc/modules/audio_coding:g722_test",
+  "//webrtc/modules/audio_coding:isac_fix_test",
+  "//webrtc/modules/audio_coding:isac_test",
+  "//webrtc/modules/audio_device/*",
+  "//webrtc/modules/audio_mixer:audio_conference_mixer",
+  "//webrtc/voice_engine:level_indicator",
 ]
 
 # These are the list of GN files that run exec_script. This whitelist exists