Add coverage pragmas

Author: A5rocks

src/trio/_core/_tests/test_thread_cache.py

@@ -225,10 +225,10 @@ def foo() -> None:
             import coverage
 
             cov = coverage.Coverage.current()
-            if cov:
+            if cov:  # pragma: no branch
                 cov.stop()
                 cov.save()
-        except ImportError:
+        except ImportError:  # pragma: no cover
             pass
 
-        os._exit(0)
+        os._exit(0)  # pragma: no cover