tests/libqtest: Move global_test wrapper function into a separate header

We want libqtest.h to become completely independent from global_qtest
(so that the wrapper functions are not used by accident anymore). As
a first step, move the wrapper functions into a separate header file.

The new header is only included from libqtest.h for now, so that there
is no difference to the users of libqtest.h yet. In the next patch, we
will switch this, so that the users of the global_qtest-related
functions will be using libqtest-single.h directly and libqtest.h
becomes completely independent of this.

Message-Id: <20190904130047.25808-6-thuth@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>