diff --git a/requirements-test.txt b/requirements-test.txt
--- a/requirements-test.txt
+++ b/requirements-test.txt
@@ -3,3 +3,4 @@
 swh.core[http] >= 0.0.61
 swh.scheduler[testing] >= 0.5.0
 swh.storage[testing]
+pytest-postgresql >= 2.1.0, < 3.0
diff --git a/swh/loader/mercurial/from_disk.py b/swh/loader/mercurial/from_disk.py
--- a/swh/loader/mercurial/from_disk.py
+++ b/swh/loader/mercurial/from_disk.py
@@ -100,6 +100,11 @@
             return default
 
 
+def in_testing_environment() -> bool:
+    """Returns whether we are in a `Pytest` session"""
+    return "PYTEST_CURRENT_TEST" in os.environ
+
+
 class HgLoaderFromDisk(BaseLoader):
     """Load a mercurial repository from a local repository."""
 
@@ -167,9 +172,16 @@
         # If set, will override the default value
         self._visit_status = None
 
-        self.old_environ = os.environ.copy()
-        os.environ.clear()
-        os.environ.update(get_minimum_env())
+        if not in_testing_environment():
+            # Do not execute in tests, since the tests already have a clean environment.
+            # Loading multiple loaders in a test will cause other tests that rely on
+            # environment variables to fail.
+            # Playing with `os.environ` is all kinds of terrible, but unfortunately
+            # `billiard` (which we use for subprocesses) does not appear to have a way
+            # of handling this any better.
+            self.old_environ = os.environ.copy()
+            os.environ.clear()
+            os.environ.update(get_minimum_env())
 
     def pre_cleanup(self) -> None:
         """As a first step, will try and check for dangling data to cleanup.
@@ -184,8 +196,10 @@
 
     def cleanup(self) -> None:
         """Last step executed by the loader."""
-        os.environ.clear()
-        os.environ.update(self.old_environ)
+        if not in_testing_environment():
+            # See comment in `__init__` about this.
+            os.environ.clear()
+            os.environ.update(self.old_environ)
 
         if self._repo_directory and os.path.exists(self._repo_directory):
             self.log.debug(f"Cleanup up repository {self._repo_directory}")
diff --git a/swh/loader/mercurial/tests/conftest.py b/swh/loader/mercurial/tests/conftest.py
--- a/swh/loader/mercurial/tests/conftest.py
+++ b/swh/loader/mercurial/tests/conftest.py
@@ -47,3 +47,4 @@
     """
     os.environ["HGPLAIN"] = ""
     os.environ["HGRCPATH"] = ""
+    os.environ["HGSKIPREPO"] = ""