Run tests under PostgreSQL (#3423)

tox.ini

@@ -1,7 +1,7 @@
 [tox]
 envlist = packaging, py27, py36, pep8, check_isort
 
-[testenv]
+[base]
 deps =
     coverage
     Twisted>=15.1
@@ -15,6 +15,15 @@ deps =
 setenv =
     PYTHONDONTWRITEBYTECODE = no_byte_code
 
+[testenv]
+deps =
+    {[base]deps}
+
+setenv =
+    {[base]setenv}
+
+passenv = *
+
 commands =
     /usr/bin/find "{toxinidir}" -name '*.pyc' -delete
     coverage run {env:COVERAGE_OPTS:} --source="{toxinidir}/synapse" \
@@ -46,6 +55,15 @@ commands =
 # )
 usedevelop=true
 
+[testenv:py27-postgres]
+usedevelop=true
+deps =
+    {[base]deps}
+     psycopg2
+setenv =
+    {[base]setenv}
+    SYNAPSE_POSTGRES = 1
+
 [testenv:py36]
 usedevelop=true
 commands =