[dualtor] Fix snmp/* tests failure on fixture teardown (#15529)

Approach
What is the motivation for this PR?
#15359 has introduced a "yield" statement inside the for loop of duthosts which is causing fixture teardown to fail with Failed: fixture function has more than one 'yield' message.

How did you do it?
Move "yield" statement out of this for loop of duthosts and do config rollback in a seperate for loop of duthosts.

How did you verify/test it?
Ran tests under snmp folder and tests are passing on Arista-7260CX3-D108C8 platform.

Any platform specific information?

tests/snmp/conftest.py

@@ -68,14 +68,15 @@ def setup_check_snmp_ready(duthosts, localhost):
             if 'LOCATION' not in snmp_location_redis_vals:
                 duthost.shell(f'sudo config snmp location add {yaml_snmp_location}')  # set snmp cli
 
-        yield
+    yield
 
+    for duthost in duthosts:
         # rollback configuration
         rollback(duthost, SETUP_ENV_CP)
 
-        # remove snmp files downloaded
-        local_command = "find ./snmp/ -type f -name 'snmp.yml' -exec rm -f {} +"
-        localhost.shell(local_command)
+    # remove snmp files downloaded
+    local_command = "find ./snmp/ -type f -name 'snmp.yml' -exec rm -f {} +"
+    localhost.shell(local_command)
 
 
 def extract_redis_keys(item):