cmd_test: check for binary operators before unary

This better mirrors the behaviour of bash, for example:

$ if test -z = -z; then echo yes; else echo no; fi
yes

This is parsed as a string comparison of "-z" and "-z", since the check
for the binary "=" operator occurs first. Without this change, the
command would be parsed as a -z test of "-", followed by a syntax error;
a trailing -z without and operand.

This is a behavioural change, but I believe any commands affected were
previously invalid or bizarely formed.

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