Fix fixture invocation order in qos/qos_sai_base.py (sonic-net#12152)

What is the motivation for this PR?
qos/test_qos_sai.py fail with the following -

failed on setup with "Failed: Not all critical processes are healthy"
The fixture dut_disable_ipv6 disables ipv6 due to which the following critical processes don't come up after swapping syncd ( fixture swapSyncd_on_selected_duts  ) -

program:snmpd
program:snmp-subagent
And this issue only hits if swapSyncd_on_selected_duts is executed after dut_disable_ipv6 and since there is no dependency between these fixtures, they can execute in any order.

How did you do it?
This change introduces a dependency between dut_disable_ipv6 & swapSyncd_on_selected_duts such that former is always executed after latter.

How did you verify/test it?
Tested on Arista-7050CX3 platform with dualtor topology.

Author: vivekverma-arista

tests/qos/qos_sai_base.py

@@ -1730,7 +1730,8 @@ def populateArpEntries(
         return
 
     @pytest.fixture(scope='class', autouse=True)
-    def dut_disable_ipv6(self, duthosts, get_src_dst_asic_and_duts, tbinfo, lower_tor_host): # noqa F811
+    def dut_disable_ipv6(self, duthosts, get_src_dst_asic_and_duts, tbinfo, lower_tor_host, # noqa F811
+                         swapSyncd_on_selected_duts):
         for duthost in get_src_dst_asic_and_duts['all_duts']:
             docker0_ipv6_addr = \
                 duthost.shell("sudo ip -6  addr show dev docker0 | grep global" + " | awk '{print $2}'")[