tests/Makefile.include: don't use TARGET_DIRS for check-tcg

tests/Makefile.include: don't use TARGET_DIRS for check-tcg

TARGET_DIRS reflects what we wanted to configure which in the normal
case is all our targets. However once meson has pared-down our target
list due to missing features we need to check the final list of
ninja-targets. This prevents check-tcg barfing on a --disable-tcg
build.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20210202134001.25738-14-alex.bennee@linaro.org>