.travis.yml: introduce TEST_BUILD_CMD and use it for check-tcg

.travis.yml: introduce TEST_BUILD_CMD and use it for check-tcg

At least for check-tcg we can split the build phase from the test
phase and do the former in parallel. While we are at it drop the V=1
for the check-tcg part as it just generates a lot more noise in the
logs.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200203090932.19147-15-alex.bennee@linaro.org>