iotests: allow test discovery before building

iotests: allow test discovery before building

The 'check' script can be invoked in "dry run" mode, in which case it
merely does test discovery and prints out all their names. Despite only
doing test discovery it still validates that the various QEMU binaries
can be found. This makes it impossible todo test discovery prior to
building QEMU. This is a desirable feature to support, because it will
let meson discover tests.

Fortunately the code in the TestEnv constructor is ordered in a way
that makes this fairly trivial to achieve. We can just short circuit
the constructor after the basic directory paths have been set.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Acked-by: Hanna Czenczek <hreitz@redhat.com>
Tested-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20230303160727.3977246-3-berrange@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20230315174331.2959-20-alex.bennee@linaro.org>