Do not hard-code database connection parameters

Rely on libpq defaults instead (overridable via environment)

tests/README.md

@@ -42,17 +42,19 @@ Make sure that you have enabled building of postgres test in CMake.
 To test the postgres provider you will need to have a database available to
 which the postgres provider can connect. The server will need to have postgis
 support enabled.
-By default the test uses the following connection options:
-    dbname='qgis_test'
-    host=localhost
-    port=5432
-    user='postgres'
-    password='postgres'
+
+By default the test uses the following connection string:
+
+    dbname=qgis_test
 
 If this does not match your setup you can set the environment variable
-QGIS_PGTEST_DB to the desired connection string.
+`QGIS_PGTEST_DB` to the desired connection string, or you can rely
+on standard libpq environment variables to tweak host, port user and
+password (PGHOST, PGPORT, PGUSER, PGPASSWORD).
 
 Please note that the database needs to be initialized using the sql-script
-    tests/testdata/provider/testdata.sql
-It takes care of activating postgis for the test database and
-creates some tables containing test data.
+
+    tests/testdata/provider/testdata_pg*.sql
+
+They take care of activating postgis for the test database and
+create some tables containing test data.

tests/src/python/test_provider_postgres.py

@@ -33,7 +33,7 @@ class TestPyQgsPostgresProvider(unittest.TestCase, ProviderTestCase):
     @classmethod
     def setUpClass(cls):
         """Run before all tests"""
-        cls.dbconn = u'dbname=\'qgis_test\' host=localhost port=5432 user=\'postgres\' password=\'postgres\''
+        cls.dbconn = u'dbname=\'qgis_test\''
         if 'QGIS_PGTEST_DB' in os.environ:
             cls.dbconn = os.environ['QGIS_PGTEST_DB']
         # Create test layers