travis-ci: Drop 'TEST_CMD'

We don't need to use TEST_CMD in order to run tests.  We need a BUILDMAN
and TOOLCHAIN variable to avoid having to duplicate logic or write some
wrapper function.  But this makes the tests harder as we add more
complex examples.

Signed-off-by: Tom Rini <trini@konsulko.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Heiko Schocher <hs@denx.de>