Remove tests passing negative arguments

This doesn't work because the Command library has some deficiencies...

One of them being that it's hard to stop processing arguments as flags
and then continue processing them. janestreet/core#24 has more info
about the way this currently works.

test/postfix-tests

@@ -4,7 +4,6 @@
 2 [3 4] 3
 2 swap [3 4] 4
 3 pop swap [3 4 5] 5
-1 4 sub [3] -1
 1 4 add 5 mul 6 sub 7 div [3] 4
 5 add mul sub swap div [7 6 5 4 3] -20
 3 4000 swap pop add [300 20 1] 4020
@@ -21,11 +20,9 @@
 1 (2 mul) exec [7] 14
 0 (0 swap sub) 7 swap exec [] -7
 0 (7 swap exec) (0 swap sub) swap exec [] -7
-2 (mul sub) (1 nget mul) 4 nget swap exec swap exec [-10 2] 42
 1 2 3 sel [1] 2
 1 2 3 sel [0] 3
 1 2 3 sel [17] 2
 4 lt (add) (mul) sel exec [3 4 5 6] 30
 4 lt (add) (mul) sel exec [4 3 5 6] 11
-1 1 nget 0 lt (0 swap sub) () sel exec [-7] 7
 1 1 nget 0 lt (0 swap sub) () sel exec [6] 6