add supported adapter list

tox.ini

@@ -1,6 +1,8 @@
 [tox]
 skipsdist = True
-envlist = lint_all, textenv
+envlist = lint_all, testenv
+# this needs to be defined on a single line.  tox has problems with multi line lists
+supported_adapters = postgres, snowflake, bigquery, redshift
 
 [testenv]
 passenv =
@@ -21,6 +23,10 @@ passenv =
     SNOWFLAKE_TEST_WAREHOUSE
     SNOWFLAKE_TEST_SCHEMA
 
+# This is required to ensure that the correct adapters are tested
+[testenv:list_adapters]
+commands = echo {[tox]supported_adapters}
+
 # Uses pytest to run tests, very basic and doesn't require dbt
 [testenv:dbt_integration_pytest]
 changedir = {toxinidir}