tests/functional: Fix bad usage of has_cmd

tests/functional: Fix bad usage of has_cmd

has_cmd returns a tuple, not a boolean value. This fixes a crash when
e.g. "tesseract" is not available in the test_m68k_nextcube test.

Reported-by: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <20240910075820.51346-1-thuth@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>