tests: exclude the expensive gnulib fts-tests

* gnulib: The only change in this gnulib update
is the tagging of the fts-tests module as longrunning,
which gnulib-tool currently implicitly excludes.
This test was seen to take about 20s and 285MB.
Reported by Assaf Gordon on space restricted VMs.

diff --git a/gnulib b/gnulib
index 58c0514fa38c16ebef9db95e215195570862f872..a20922f1055979e45de0a42a091f08009acfa99a 160000 (submodule)
--- a/gnulib
+++ b/gnulib
@@ -1 +1 @@
-Subproject commit 58c0514fa38c16ebef9db95e215195570862f872
+Subproject commit a20922f1055979e45de0a42a091f08009acfa99a