cmd_test: use table lookup for parsing

do_test() currently uses strcmp() twice to determine which operator is
present; once to determine how many arguments the operator needs, then
a second time to actually decode the operator and implement it.

Rewrite the code so that a table lookup is used to translate the operator
string to an integer, and use a more efficient switch statement to decode
and execute the operator.

This approach also acts as enablement for the following patches.

This patch should introduce no behavioural change.

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