test(di): avoid pytest loader when importing modules (#12812)

We avoid the use of the pytest loader when importing a module in one of
the DI tests for decorated function discovery. Because the current
implementation of the pytest loader does not implement the get_code
method, we cannot wrap around it to get the code object for the module
before it is executed. As a workaround in tests, we temporarily remove
the pytest loader so that we can import the module under "normal"
conditions. Once the test is done we restore the state of the
sys.meta_path list.

## Checklist
- [x] PR author has checked that all the criteria below are met
- The PR description includes an overview of the change
- The PR description articulates the motivation for the change
- The change includes tests OR the PR description describes a testing
strategy
- The PR description notes risks associated with the change, if any
- Newly-added code is easy to change
- The change follows the [library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
- The change includes or references documentation updates if necessary
- Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist
- [ ] Reviewer has checked that all the criteria below are met 
- Title is accurate
- All changes are related to the pull request's stated goal
- Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes
- Testing strategy adequately addresses listed risks
- Newly-added code is easy to change
- Release note makes sense to a user of the library
- If necessary, author has acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment
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[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)

Author: P403n1x87

tests/debugging/function/test_discovery.py

@@ -1,10 +1,25 @@
+import sys
+
 import pytest
 
 from ddtrace.debugging._function.discovery import FunctionDiscovery
 from ddtrace.internal.module import ModuleWatchdog
 import tests.submod.stuff as stuff
 
 
+@pytest.fixture
+def no_pytest_loader():
+    from _pytest.assertion.rewrite import AssertionRewritingHook
+
+    i = next(i for i, hook in enumerate(sys.meta_path) if isinstance(hook, AssertionRewritingHook))
+    pytest_loader = sys.meta_path.pop(i)
+
+    try:
+        yield
+    finally:
+        sys.meta_path.insert(i, pytest_loader)
+
+
 @pytest.fixture
 def stuff_discovery():
     return FunctionDiscovery.from_module(stuff)
@@ -124,18 +139,20 @@ def test_property_non_function_getter(stuff_discovery):
         stuff_discovery.by_name("PropertyStuff.foo")
 
 
-def test_custom_decorated_stuff():
+def test_custom_decorated_stuff(no_pytest_loader):
     class DiscoveryModuleWatchdog(ModuleWatchdog):
         def transform(self, code, module):
             return FunctionDiscovery.transformer(code, module)
 
     DiscoveryModuleWatchdog.install()
 
-    import tests.submod.custom_decorated_stuff as custom_decorated_stuff
+    try:
+        import tests.submod.custom_decorated_stuff as custom_decorated_stuff
 
-    fd = FunctionDiscovery.from_module(custom_decorated_stuff)
+        fd = FunctionDiscovery.from_module(custom_decorated_stuff)
 
-    (home,) = fd.at_line(17)
-    assert home.__qualname__ == "home"
+        (home,) = fd.at_line(17)
+        assert home.__qualname__ == "home"
 
-    DiscoveryModuleWatchdog.uninstall()
+    finally:
+        DiscoveryModuleWatchdog.uninstall()