meta-agl-test:add new feature 'agl-test'

'agl-test' is used to compile the test sets and test framework into the
image, make it more convenient to test target.

Now it's only support the target agl-demo-platform(ivi)

Enable the  `agl-test` AGL feature when setting up your build environment
with aglsetup.sh.

This will add the `packagegroup-agl-extend-test` packagegroup to the image,
For specific packages to be added, please refer to the file:
    ./recipes-platform/packagegroup/packagegroup-agl-extend-test.bb

For running test using 'agl-test', refer to:
    https://git.automotivelinux.org/src/agl-test-framework/tree/README

Bug-AGL: SPEC-4345

Signed-off-by: duerpei <duep.fnst@fujitsu.com>
Change-Id: Ic217bff672f3dae1ac3abc18c01ba802ebcb8fe1