tests: replace [[]] bashism with portable [] invocation

In this case nothing fancy is required.

Differential Revision: https://phab.mercurial-scm.org/D7596

tests/test-amend.t

@@ -493,7 +493,7 b' Modifying a file while the editor is ope'
   $ sleep 1
   $ echo delta >> foo
   $ sleep 3
-  $ if (hg diff -c . | grep 'delta' >/dev/null) || [[ -n "$(hg status)" ]]; then
+  $ if (hg diff -c . | grep 'delta' >/dev/null) || [ -n "$(hg status)" ]; then
   >   echo "OK."
   > else
   >   echo "Bug detected. 'delta' is not part of the commit OR the wdir"