fix check_dut_asic_type fixture index error (sonic-net#14830)

Updated the rand_one_dut_hostname and selected_rand_dut fixtures to
set rand_one_dut_hostname_var whenever needed, specifically in the case
the fixture is dynamically loaded. Removed old method of setting
rand_one_dut_hostname_var.

tests/conftest.py

@@ -493,6 +493,8 @@ def rand_one_dut_hostname(request):
     """
     """
     global rand_one_dut_hostname_var
+    if rand_one_dut_hostname_var is None:
+        set_rand_one_dut_hostname(request)
     return rand_one_dut_hostname_var
 
 
@@ -507,6 +509,8 @@ def rand_selected_dut(duthosts, rand_one_dut_hostname):
 @pytest.fixture(scope="module")
 def selected_rand_dut(request):
     global rand_one_dut_hostname_var
+    if rand_one_dut_hostname_var is None:
+        set_rand_one_dut_hostname(request)
     return rand_one_dut_hostname_var
 
 
@@ -1699,12 +1703,6 @@ def pytest_generate_tests(metafunc):        # noqa E302
     if dut_fixture_name and "selected_dut" in metafunc.fixturenames:
         metafunc.parametrize("selected_dut", duts_selected, scope="module", indirect=True)
 
-    # When rand_one_dut_hostname used and select a dut for test, initialize rand_one_dut_hostname_var
-    # rand_one_dut_hostname and rand_selected_dut will use this variable for setup test case
-    # selected_rand_dut will use this variable for setup TACACS
-    if "rand_one_dut_hostname" in metafunc.fixturenames:
-        set_rand_one_dut_hostname(metafunc)
-
     if "enum_dut_portname" in metafunc.fixturenames:
         metafunc.parametrize("enum_dut_portname", generate_port_lists(metafunc, "all_ports"))