tests: enable subprocess coverage

Subprocess coverage used to be provided by default by pytest-cov,
however that was removed in newer versions in favor of using the
coverage patch functionality. Use the coverage patch functionality to
re-enable coverage of subprocesses, re-enabling coverage for the remote
tests.

Signed-off-by: Rouven Czerwinski <rouven.czerwinski@linaro.org>

Author: Emantor

.coveragerc

@@ -1,6 +1,7 @@
 [run]
 omit =
     labgrid/remote/generated/*
+patch = subprocess
 
 [report]
 exclude_lines =
@@ -11,4 +12,4 @@ exclude_lines =
     raise AssertionError
     raise NotImplementedError
     if 0:
-    if __name__ == .__main__.:
+    if __name__ == .__main__.: