build: disarm the TCG unit test trap

Developers sometimes mistakenly run 'make test' instead of 'make check'.
'make test' triggers the ancient, unmaintained tcg unit tests in
tests/tcg/Makefile which have long since ceased compiling.

Even if someone fixes the TCG tests, it makes little sense to put
them in a 'make test' target, rather they should be 'make check-tcg',
possibly wired up as a dependency of 'make check'.

In the meantime, this patch disarms the 'make test' trap by simply
deleting it so users get an immediate error. This should be enough
for them to remember to type 'make check' instead (or 'make help'
to learn). It also deletes 'make speed' which is another route
into the tcg tests.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Kashyap Chamarthy <kchamart@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb@linux.vnet.ibm.com>
Message-id: 20171121142538.22072-1-berrange@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
