Revert "Run test_port_security_macspoofing_port slow tests serially"

This reverts commit 33e4e24df59605a9e2187f410f470a11d0549c8f.

Reason for revert: Test fixed with [1].
[1] https://review.opendev.org/c/openstack/tempest/+/889713

Change-Id: Ied75aa4c3332a99bc10bcb65d4624a5cafd56f8b
diff --git a/tox.ini b/tox.ini
index 027442a..47ef5eb 100644
--- a/tox.ini
+++ b/tox.ini
@@ -328,15 +328,10 @@
 setenv = {[tempestenv]setenv}
 deps = {[tempestenv]deps}
 # The regex below is used to select the slow tagged tests:
-regex1 = '(.*\[.*\bslow\b.*\])'
-regex2 = 'test_port_security_macspoofing_port'
-# The regex below is used to run slow test in parallel except
-# test_port_security_macspoofing_port which fail many times
-# while running in parallel
+regex = '\[.*\bslow\b.*\]'
 commands =
     find . -type f -name "*.pyc" -delete
-    tempest run --regex {[testenv:slow]regex1} --exclude-regex {[testenv:slow]regex2} {posargs}
-    tempest run --combine --serial --regex {[testenv:slow]regex2} {posargs}
+    tempest run --regex {[testenv:slow]regex} {posargs}
 
 [testenv:multinode]
 envdir = .tox/tempest