[net_test_utils] correct the return path of stop_tcpdump

Fix a issue that the return path of stop_tcpdump was
changed to directory accidentally

Bug: 171472638
Test: Local
Change-Id: Idcd2e2de16a9e465a2c40c99daf9814aaefac9d7
diff --git a/acts/framework/acts/test_utils/net/net_test_utils.py b/acts/framework/acts/test_utils/net/net_test_utils.py
index 93e62a7..bfdc64d 100644
--- a/acts/framework/acts/test_utils/net/net_test_utils.py
+++ b/acts/framework/acts/test_utils/net/net_test_utils.py
@@ -290,8 +290,8 @@
 
     file_name = "%s/tcpdump_%s_%s.pcap" % (TCPDUMP_PATH, ad.serial, test_name)
     ad.log.info("tcpdump file is %s", file_name)
-    cmd = "adb -s {} shell tcpdump -i any -W 100 -C 50 -s0 -w {}".format(
-            ad.serial, file_name)
+    cmd = "adb -s {} shell tcpdump -i any -s0 -w {}".format(ad.serial,
+                                                            file_name)
     try:
         return start_standing_subprocess(cmd, 5)
     except Exception:
@@ -330,7 +330,8 @@
         ad.adb.pull("%s/. %s" % (TCPDUMP_PATH, log_path),
                 timeout=adb_pull_timeout)
         ad.adb.shell("rm -rf %s/*" % TCPDUMP_PATH, ignore_status=True)
-        return log_path
+        file_name = "tcpdump_%s_%s.pcap" % (ad.serial, test_name)
+        return "%s/%s" % (log_path, file_name)
     return None
 
 def start_tcpdump_gce_server(ad, test_name, dest_port, gce):