fix error message when completing backquote command substitution; fix test builtin with double negation

tests/RUN-ONE-TEST

@@ -1,4 +1,4 @@
-BUILD_DIR=/usr/local/build/chet/bash/bash-current
+BUILD_DIR=/usr/local/build/bash/bash-current
 THIS_SH=$BUILD_DIR/bash
 PATH=$PATH:$BUILD_DIR
 

tests/test.right

@@ -220,6 +220,8 @@ t ! -z "$z"
 0
 t ! -n "$z"
 1
+t ! ! "$z"
+0
 t "$zero"
 1
 t ! "$zero"
@@ -272,7 +274,7 @@ b ( 1 = 2
 2
 ./test.tests: line 26: test: too many arguments
 2
-./test.tests: line 431: [: missing `]'
+./test.tests: line 434: [: missing `]'
 2
 ./test.tests: line 26: test: (: unary operator expected
 2

tests/test.tests

@@ -358,6 +358,9 @@ t ! -z "$z"
 echo 't ! -n "$z"'
 t ! -n "$z"
 
+echo 't ! ! "$z"'
+t ! ! "$z"
+
 zero=
 echo 't "$zero"'
 t "$zero"