tests/functional: introduce some helpful decorators

tests/functional: introduce some helpful decorators

Reduce repeated boilerplate with some helper decorators:

 @skipIfNotPlatform("x86_64", "aarch64")

  => Skip unless the build host platform matches

 @skipIfMissingCommands("mkisofs", "losetup")

  => Skips unless all listed commands are found in $PATH

 @skipIfMissingImports("numpy", "cv2")

  => Skips unless all listed modules can be imported

 @skipFlakyTest("https://gitlab.com/qemu-project/qemu/-/issues/NNN")

  => Skips unless env var requests flaky tests with the
     reason documented in the referenced gitlab bug

 @skipBigData

  => Skips unless env var permits tests creating big data files

 @skipUntrustedTest

  => Skips unless env var permits tests which are potentially
     dangerous to the host

Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-ID: <20241217155953.3950506-8-berrange@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>