Bug 1665494 - In mach python-test, don't install requirements.txt in parallel r=ahal

If you set a `DEFAULT` `requirements` file in a Python test manifest, the current implementation of `python-test` will try (and usually fail) to install the file once for every test file in parallel. Instead, initializing the environment should be done once when the command starts up.

Differential Revision: https://phabricator.services.mozilla.com/D90475

python/mach_commands.py

@@ -181,6 +181,14 @@ class MachCommands(MachCommandBase):
         if extra:
             os.environ['PYTEST_ADDOPTS'] += " " + " ".join(extra)
 
+        installed_requirements = set()
+        for test in tests:
+            if (test.get('requirements') and
+                test['requirements'] not in installed_requirements):
+                self.virtualenv_manager.install_pip_requirements(
+                    test['requirements'], quiet=True)
+                installed_requirements.add(test['requirements'])
+
         if exitfirst:
             sequential = tests
             os.environ['PYTEST_ADDOPTS'] += " -x"
@@ -234,9 +242,6 @@ class MachCommands(MachCommandBase):
     def _run_python_test(self, test):
         from mozprocess import ProcessHandler
 
-        if test.get('requirements'):
-            self.virtualenv_manager.install_pip_requirements(test['requirements'], quiet=True)
-
         output = []
 
         def _log(line):