support/testing/: change python-gobject to use prebuilt kernel

support/testing/: change python-gobject to use prebuilt kernel

Currently, we build a complete AArch64 system, including a kernel. This
can take quite some time.

Switch to an armv7 system, which allows us to use one a prebuilt kernel,
thus significantly reducing the test time.

Signed-off-by: Adam Duskett <Aduskett@gmail.com>
[yann.morin.1998@free.fr:
  - use a standalone config; don't inherit from TestPythonPackageBase
  - use the default external toolchain (ARM, not Linaro)
  - rewrite commit log
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>