add hush tests for parameter expansion

shell/hush_test/hush-vars/param_expand_alt.tests

@@ -0,0 +1,22 @@
+# first try some invalid patterns (do in subshell due to parsing error)
+hush -c 'echo ${+}  ; echo moo'
+hush -c 'echo ${:+} ; echo moo'
+
+# now some funky ones
+echo _${#+} _${#:+}
+
+# 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}