test-sysroot: Skip on s390x by default

This test regularly fails on the buildds, but I cannot reproduce the
failure on a porterbox.

Bug: ostreedev/ostree#2527
Bug-Debian: https://bugs.debian.org/1025532
Forwarded: not-needed

Gbp-Pq: Topic debian
Gbp-Pq: Name test-sysroot-Skip-on-s390x-by-default.patch

tests/test-sysroot.js

@@ -38,6 +38,11 @@ function libtestExec(shellCode) {
     proc.wait_check(null);
 }
 
+if (GLib.getenv('DEB_HOST_ARCH') === 's390x' && !GLib.getenv('DEB_ALLOW_FLAKY_TESTS')) {
+    print('1..0 # SKIP https://bugs.debian.org/1025532');
+    imports.system.exit(0);
+}
+
 print('1..1')
 
 libtestExec('setup_os_repository archive syslinux');