hush: fix $$ handling

diff --git a/shell/hush_test/hush-misc/pid.tests b/shell/hush_test/hush-misc/pid.tests
new file mode 100755
index 0000000..eaeaa71
--- /dev/null
+++ b/shell/hush_test/hush-misc/pid.tests
@@ -0,0 +1 @@
+test `(echo $$)` = `echo $$`; echo $?