dev-python/pytest-cov-2.3.1: Fix test suite failure

Install the package properly for testing with distutils_install_for_testing.

Disable tests which still fail. These are:

* test_central_subprocess_change_cwd_with_pythonpath, because it tampers with
  PYTHONPATH (which was adjusted by distutils_install_for_testing).

* test_dist_missing_data, because it tries to access the internet which fails
  when the feature network-sandbox is enabled.

diff -ur a/dev-python/pytest-cov/pytest-cov-2.3.1.ebuild b/dev-python/pytest-cov/pytest-cov-2.3.1.ebuild
--- a/dev-python/pytest-cov/pytest-cov-2.3.1.ebuild
+++ b/dev-python/pytest-cov/pytest-cov-2.3.1.ebuild
@@ -44,13 +44,26 @@ PATCHES=(
 	"${FILESDIR}/fix-for-deprecation-warnings.patch"
 )
 
+src_prepare() {
+	default
+
+	# Mark tests which tamper with PYTHONPATH with xfail since they will fail
+	# inside a portage build. Bug #597708
+	# Also mark tests which need internet access (which fails with feature
+	# network-sandbox enabled).
+	sed -i \
+		-e '/^def test_central_subprocess_change_cwd_with_pythonpath/ i\@pytest.mark.xfail(reason="Changing PYTHONPATH causes trouble inside of an emerge process")' \
+		-e '/^def test_dist_missing_data/ i\@pytest.mark.xfail(reason="internet access fails with feature network-sandbox enabled")' \
+		tests/test_pytest_cov.py
+}
+
 python_compile_all() {
 	use doc && sphinx-build -b html docs _build/html
 }
 
 python_test() {
-	PYTHONPATH="${S}/tests:${BUILD_DIR}/lib" \
-		PYTEST_PLUGINS=${PN/-/_} \
+	distutils_install_for_testing
+	PYTHONPATH="${S}/tests:${BUILD_DIR}/lib:${PYTHONPATH}" \
 		py.test -v -v -x || die "Tests failed under ${EPYTHON}"
 }