Recover coverage from subprocesses during unit tests (#15354)

* Recover coverage from subprocesses during unit tests

pytest.ini

@@ -0,0 +1,10 @@
+# content of pytest.ini
+[pytest]
+addopts = --durations=20 -ra
+testpaths = lib/spack/spack/test
+python_files = *.py
+markers =
+  db: tests that require creating a DB
+  network: tests that require access to the network
+  maybeslow: tests that may be slow (e.g. access a lot the filesystem, etc.)
+  regression: tests that fix a reported bug