tests/functional: Fix the aarch64_tcg_plugins test

tests/functional: Fix the aarch64_tcg_plugins test

Unfortunately, this test had not been added to meson.build, so we did
not notice a regression: Looking for 'Kernel panic - not syncing: VFS:'
as the indication for the final boot state of the kernel was a bad
idea since 'Kernel panic - not syncing' is the default failure
message of the LinuxKernelTest class, and since we're now reading
the console input byte by byte instead of linewise (see commit
cdad03b74f75), the failure now triggers before we fully read the
success string. Let's fix this by simply looking for the previous
line in the console output instead.

Also, replace the call to cancel() - this was only available in the
Avocado framework. In the functional framework, we must use skipTest()
instead. While we're at it, also fix the TODO here by looking for the
exact error and only skip the test if the plugins are not available.

Fixes: 3abc545e66 ("tests/functional: Convert the tcg_plugins test")
Fixes: cdad03b74f ("tests/functional: rewrite console handling to be bytewise")
Message-ID: <20250123083625.1498495-1-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>