cmd_test: implement ! on sub-expressions

cmd_test: implement ! on sub-expressions

Currently, ! can only be parsed as the first operator in an expression.
This prevents the following from working:

$ if test ! ! 1 -eq 1; then echo yes; else echo no; fi
yes
$ if test ! 1 -eq 2 -a ! 3 -eq 4; then echo yes; else echo no; fi
yes

Fix this by parsing ! like any other operator, and and handling it
similarly to -a and -o.

Signed-off-by: Stephen Warren <swarren@nvidia.com>