Use OS_TEST_PATH evironment variable to define tests location

Change-Id: Ia8b4d6e48804c99167429e3535fa4adfc9e880f4

.stestr.conf

@@ -1,3 +1,3 @@
 [DEFAULT]
-test_path=./ovn_bgp_agent/tests
+test_path=${OS_TEST_PATH:-./ovn_bgp_agent/tests/unit}
 top_dir=./

tox.ini

@@ -16,7 +16,7 @@ setenv =
    OS_TEST_TIMEOUT=60
 deps = -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
        -r{toxinidir}/test-requirements.txt
-commands = stestr run --exclude-regex ".tests.functional" {posargs}
+commands = stestr run {posargs}
 
 [testenv:pep8]
 commands = flake8 {posargs}
@@ -30,9 +30,10 @@ setenv = {[testenv]setenv}
          OVS_SRCDIR={envdir}/src/ovn/ovs
          VTEP_SRCDIR={envdir}/src/ovn/ovs/vtep
          OVN_BRANCH={env:OVN_BRANCH:}
+         OS_TEST_PATH=./ovn_bgp_agent/tests/functional
 commands =
   bash {toxinidir}/tools/setup-ovs.sh
-  stestr run --exclude-regex ".tests.unit" {posargs}
+  {[testenv]commands}
 allowlist_externals = bash
 
 [testenv:cover]
@@ -40,7 +41,7 @@ setenv =
     VIRTUAL_ENV={envdir}
     PYTHON=coverage run --source ovn_bgp_agent --parallel-mode
 commands =
-    stestr run --exclude-regex ".tests.functional" {posargs}
+    {[testenv]commands}
     coverage combine
     coverage html -d cover
     coverage xml -o cover/coverage.xml