Many tests need to access assets stored on remote sites. We don't want
to download these during test execution when run by meson, since this
risks hitting test timeouts when data transfers are slow.
Add support for pre-emptive caching of assets by setting the env var
QEMU_TEST_PRECACHE to point to a timestamp file. When this is set,
instead of running the test, the assets will be downloaded and saved
to the cache, then the timestamp file created.
A meson custom target is created as a dependency of each test suite
to trigger the pre-emptive caching logic before the test runs.
When run in caching mode, it will locate assets by looking for class
level variables with a name prefix "ASSET_", and type "Asset".
At the ninja level
ninja test --suite functional
will speculatively download any assets that are not already cached,
so it is advisable to set a timeout multiplier.
QEMU_TEST_NO_DOWNLOAD=1 ninja test --suite functional
will fail the test if a required asset is not already cached
nina check-func-precache