Add Javascript mocha test snippets

snippets/js2-mode/context.yasnippet

+# -*- mode: snippet -*-
+# contributor: ßingen
+# key: ctx
+# name: context test unit
+# --
+context('$1', () => {
+  $0
+})

snippets/js2-mode/it.yasnippet

+# -*- mode: snippet -*-
+# contributor: ßingen
+# key: it
+# name: it test unit
+# --
+it('$1', async () => {
+  $0
+})