tests: Add virnetdevtestdata to EXTRA_DIST

In one of my previous commits (49ed6cff) I've introduced a test
among with some files stored under virnetdevtestdata folder.
While this works perfectly within a git tree, the folder was not
getting into .tar.gz and therefore the dist-check would fail.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
(cherry picked from commit 598f3fdd)

tests/Makefile.am

@@ -132,6 +132,7 @@ EXTRA_DIST =		\
 	vboxsnapshotxmldata \
 	virsh-uriprecedence \
 	virfiledata \
+	virnetdevtestdata \
 	virpcitestdata \
 	virscsidata \
 	virusbtestdata \