oeqa: do not fail when sdk package is not available

oeqa: do not fail when sdk package is not available

Prior to commits:
* 11277efd057685558a744e98082b5709e849dd2a
* d0e8b83d05957b1f22d08582e364afa4b522801e
the tests were skipped if package was not available.
Now the code calls function ensure_host_package which says
"try to sdk-install missing dependencies", however in fact for sdkext it
causes a failure if the installation is not available.

Since maturin is not installed in any image, it cannot be installed
unless it's downloaded from sstate-cache mirror populated by a world
build. These builds are however now not done for powerpc and mips.
IT also does not work in local builds without sstate-cache mirror.

Fix this by skipping the test if the package cannot be installed to
match the original behavior before those commits.

Signed-off-by: Peter Marko <peter.marko@siemens.com>
Cc: Ross Burton <ross.burton@arm.com>
Cc: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>