command: test: Move into the cmd suite

command: test: Move into the cmd suite

The command test was the very first test written in U-Boot, some 12
years ago. It predates the unit-test subsystem and was never converted
over.

There is no particular need for the command test to have its own
command. It is also confusing to have it separate from the normal test
suites. At present this test is not run in CI.

Move it into the cmd suite instead, updating it to become a unit test.
One of the checks is dropped to avoid an error.

Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Tom Rini <trini@konsulko.com> # rpi_3, rpi_4, rpi_arm64, am64x_evm_a53, am64-sk