tests/meson.build: create testlists at build time

Let's dynamically create testlists during build time, as this
can speed up a lot the validation check for testplan.

Ideally, all it would need to to that would be to use this
logic at the end of tests/meson.build:

    foreach testexe : test_executables
        prog = testexe.name()

        output = prog.split('/').get(-1) + '.testlist'
        custom_target(output,
                     build_by_default : true,
                     command : [ testexe, '--show-testlist'],
                     capture : true,
                     output : output)

Unfortunately, this requies Meson >= 0.54. So, we need to add
one custom_target per executable() call.

Reviewed-by: Kamil Konieczny <kamil.konieczny@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>