tests: Skip 'guix-container-environment.sh' when support is missing.

Fixes <http://bugs.gnu.org/21410>.
Reported by Alex Vong <alexvong1995@gmail.com>.

* tests/guix-environment-container.sh: Exit with 77 when
'((@@ (guix scripts environment) assert-container-features))' fails.

tests/guix-environment-container.sh

@@ -24,6 +24,12 @@ set -e
 
 guix environment --version
 
+if ! guile -c '((@@ (guix scripts environment) assert-container-features))'
+then
+    # User containers are not supported; skip this test.
+    exit 77
+fi
+
 tmpdir="t-guix-environment-$$"
 trap 'rm -r "$tmpdir"' EXIT