pytest-postgresql requires psycopg

See https://github.com/ClearcodeHQ/pytest-postgresql#how-to-use

Fix failing queue_worker unit tests when psycopg2 is installed
(otherwise they are skipped):

```
$ pytest test/unit/app/queue_worker/test_database_heartbeat.py::test_database_heartbeat[postgres_app]
...
_________________________________________________________________________ ERROR at setup of test_database_heartbeat[postgres_app] _________________________________________________________________________

request = <SubRequest 'database_app' for <Function test_database_heartbeat[postgres_app]>>

    @pytest.fixture(params=["postgres_app", "sqlite_app", "sqlite_rabbitmq_app"])
    def database_app(request):
        if request.param == "postgres_app":
            if not which("initdb"):
                pytest.skip("initdb must be on PATH for postgresql fixture")
            if not psycopg2:
                pytest.skip("psycopg2 must be installed for postgresql fixture")
        if request.param == "sqlite_rabbitmq_app":
            if not os.environ.get("GALAXY_TEST_AMQP_INTERNAL_CONNECTION"):
                pytest.skip("rabbitmq tests will be skipped if GALAXY_TEST_AMQP_INTERNAL_CONNECTION env var is unset")
>       return request.getfixturevalue(request.param)

test/unit/app/queue_worker/conftest.py:67:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
.venv/lib/python3.10/site-packages/pytest_postgresql/factories/process.py:147: in postgresql_proc_fixture
    with DatabaseJanitor(
.venv/lib/python3.10/site-packages/pytest_postgresql/janitor.py:54: in __init__
    check_for_psycopg()
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

    def check_for_psycopg() -> None:
        """
        Function checks whether psycopg was imported.

        Raises ImportError if not.
        """
        if not psycopg:
>           raise ImportError("No module named psycopg. Please install psycopg.")
E           ImportError: No module named psycopg. Please install psycopg.

.venv/lib/python3.10/site-packages/pytest_postgresql/compat.py:35: ImportError
```

Also, remove check for `initdb` in PATH, because on Ubuntu it's installed
in `/usr/lib/postgresql/VERSION/bin/initdb` .

test/unit/app/queue_worker/conftest.py

@@ -4,13 +4,17 @@
 
 import pytest
 
+try:
+    import psycopg
+except ImportError:
+    psycopg = None
+
 try:
     import psycopg2
 except ImportError:
     psycopg2 = None
 
 from galaxy.app_unittest_utils import galaxy_mock
-from galaxy.util import which
 
 
 def create_base_test(connection, amqp_type: str, amqp_connection: Optional[str] = None):
@@ -48,9 +52,6 @@ def create_app():
 def postgres_app(postgresql_proc):
     connection = "postgresql://{p.user}@{p.host}:{p.port}/".format(p=postgresql_proc)
 
-    if not psycopg2:
-        pytest.skip("psycopg2 must be installed for postgresql fixture")
-
     def create_app():
         return create_base_test(connection, amqp_type="postgres")
 
@@ -60,12 +61,11 @@ def create_app():
 @pytest.fixture(params=["postgres_app", "sqlite_app", "sqlite_rabbitmq_app"])
 def database_app(request):
     if request.param == "postgres_app":
-        if not which("initdb"):
-            pytest.skip("initdb must be on PATH for postgresql fixture")
+        if not psycopg:
+            pytest.skip("psycopg must be installed for postgresql_proc fixture")
+        if not psycopg2:
+            pytest.skip("psycopg2 must be installed for database_app fixture")
     if request.param == "sqlite_rabbitmq_app":
         if not os.environ.get("GALAXY_TEST_AMQP_INTERNAL_CONNECTION"):
             pytest.skip("rabbitmq tests will be skipped if GALAXY_TEST_AMQP_INTERNAL_CONNECTION env var is unset")
-    try:
-        return request.getfixturevalue(request.param)
-    except ImportError:
-        pytest.skip("psycopg2 must be installed for postgresql fixture")
+    return request.getfixturevalue(request.param)