tests/tcg: prepare Makefile.prereqs at configure time

List the dependencies of the build-tcg-tests-* and run-tcg-tests-*
targets in a Makefile fragment, without going through Makefile.prereqs's
"parsing" of config-*.mak.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220401141326.1244422-13-pbonzini@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220419091020.3008144-16-alex.bennee@linaro.org>