oeqa/sdk/context: fix multilib handling bug in hasTargetPackage

oeqa/sdk/context: fix multilib handling bug in hasTargetPackage

hasTargetPackage has some logic to automatically multilibify package
names if needed (for example, so that gtk+3 becomes lib32-gtk+3).

Due to a logic bug if multilib was True but there were no multilibs
configured then this prepended "-" to the package name, which won't
exist.  This resulted in tests being skipped as the dependent packages
are not installed.

Solve this by only prepending to the package name if requested and if a
multilib environment has been detected.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>