cmd: test: fix handling of single-argument form of test

cmd: test: fix handling of single-argument form of test

POSIX states that

  0 arguments:
      Exit false (1).
  1 argument:
      Exit true (0) if $1 is not null; otherwise, exit false.

and at least bash and busybox sh behave that way.

The current 'argc < 3' does the right thing for a non-existing or
empty argv[1], but not for a non-empty argv[1]. Fix that and add
corresponding test cases.

Signed-off-by: Rasmus Villemoes <ravi@prevas.dk>
Tested-by: Anshul Dalal <anshuld@ti.com>