Fix test failure in test_cont_link_flap.py (sonic-net#8128)

Fixes test failure in test_cont_link_flap.py caused by sonic-net#7968

What is the motivation for this PR?
Test failed with error:
File "/azp/_work/6/s/tests/platform_tests/link_flap/test_cont_link_flap.py", line 145, in test_cont_link_flap
incr_redis_memory = float(end_time_redis_memory) - float(start_time_redis_memory)
ValueError: could not convert string to float: '\1'

How did you do it?
Correct reg express.

How did you verify/test it?
Manually run test case: test_cont_link_flap.py

Author: wsycqyz

tests/platform_tests/link_flap/test_cont_link_flap.py

@@ -54,7 +54,7 @@ def test_cont_link_flap(self, request, duthosts, nbrhosts, enum_rand_one_per_hws
 
         # Record Redis Memory at start
         start_time_redis_memory = duthost.shell(
-            r"redis-cli info memory | grep used_memory_human | sed -e 's/.*:\(.*\)M/\\1/'")["stdout"]
+            r"redis-cli info memory | grep used_memory_human | sed -e 's/.*:\(.*\)M/\1/'")["stdout"]
         logging.info("Redis Memory: %s M", start_time_redis_memory)
 
         # Record ipv4 route counts at start
@@ -137,7 +137,7 @@ def test_cont_link_flap(self, request, duthosts, nbrhosts, enum_rand_one_per_hws
 
         # Record Redis Memory at end
         end_time_redis_memory = duthost.shell(
-            r"redis-cli info memory | grep used_memory_human | sed -e 's/.*:\(.*\)M/\\1/'")["stdout"]
+            r"redis-cli info memory | grep used_memory_human | sed -e 's/.*:\(.*\)M/\1/'")["stdout"]
         logging.info("Redis Memory at start: %s M", start_time_redis_memory)
         logging.info("Redis Memory at end: %s M", end_time_redis_memory)
 

tests/platform_tests/link_flap/test_link_flap.py

@@ -37,7 +37,7 @@ def test_link_flap(request, duthosts, rand_one_dut_hostname, tbinfo, fanouthosts
 
     # Record Redis Memory at start
     start_time_redis_memory = duthost.shell(
-        r"redis-cli info memory | grep used_memory_human | sed -e 's/.*:\(.*\)M/\\1/'")["stdout"]
+        r"redis-cli info memory | grep used_memory_human | sed -e 's/.*:\(.*\)M/\1/'")["stdout"]
     logger.info("Redis Memory: %s M", start_time_redis_memory)
 
     # Make Sure Orch CPU < orch_cpu_threshold before starting test.
@@ -76,7 +76,7 @@ def test_link_flap(request, duthosts, rand_one_dut_hostname, tbinfo, fanouthosts
 
     # Record Redis Memory at end
     end_time_redis_memory = duthost.shell(
-        r"redis-cli info memory | grep used_memory_human | sed -e 's/.*:\(.*\)M/\\1/'")["stdout"]
+        r"redis-cli info memory | grep used_memory_human | sed -e 's/.*:\(.*\)M/\1/'")["stdout"]
     logger.info("Redis Memory at start: %s M", start_time_redis_memory)
     logger.info("Redis Memory at end: %s M", end_time_redis_memory)