oeqa/bblayers.py: add tests for adding new built-in fragments

oeqa/bblayers.py: add tests for adding new built-in fragments

Add discussed in [1], our best option for customizing built-in fragments
is to pass them from a layer configuration. In short, the reason is that
our statement must be parsed before the addfragments call is parsed.We
also have to use the :append override as using += would override the
original definition of OE_FRAGMENTS_BUILTIN (since it uses a ?=
assignment).

Provide a test case for customizing built-in fragments with
meta-selftest.

[1]: https://lore.kernel.org/yocto-docs/20250925-fragments-v1-0-c9f747361fb2@bootlin.com/T/#m9f7c9f110c084eba17e0f64d8b2ac7a88af3f38e

Cc: Alexander Kanavin <alex.kanavin@gmail.com>
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>