testsdk: Handle minimal eSDK and avoid download costs

When using a minimal eSDK, testing currently fails as the sdk isn't
populated. We therefore setup the eSDK under test to point at local
sstate and execute a command to ensure the toolchain is populated
since most of the tests depend on this being present.

At the same time, add in a link to DL_DIR through own-mirrors so
that tests which fetch source (e.g. the kernel module one) can
use the local stash. This cuts test execution of the kernel module
test from 2000s to 120s.

We did try using DL_DIR directly but that causes uninative issues
requiring other workarounds so own-mirrors is neater.

Together these fixes unbreak eSDK testing on the autobuilder.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

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