add hush tests for parameter expansion

shell/hush_test/hush-vars/param_expand_default.tests

@@ -0,0 +1,22 @@
+# first try some invalid patterns (do in subshell due to parsing error)
+hush -c 'echo ${-}'
+hush -c 'echo ${:-}'
+
+# now some funky ones
+echo _${#-} _${#:-}
+
+# now some valid ones
+set --
+echo _$1 _${1-} _${1:-} _${1-word} _${1:-word}
+
+set -- aaaa
+echo _$1 _${1-} _${1:-} _${1-word} _${1:-word}
+
+unset f
+echo _$f _${f-} _${f:-} _${f-word} _${f:-word}
+
+f=
+echo _$f _${f-} _${f:-} _${f-word} _${f:-word}
+
+f=fff
+echo _$f _${f-} _${f:-} _${f-word} _${f:-word}