tests: make the test context persistent

tests: make the test context persistent

The TESTs in ivi_layout-test.c may have several server-side parts
(RUNNER_TEST in ivi_layout-test-plugin.c) each. Sometimes we need to
carry state from one RUNNER_TEST to another within one TEST, but not
across multiple TESTs. The correct lifetime of that state would be the
lifetime (and identity) of the runner_resource, as one TEST creates and
uses at most one weston_test_runner during its lifetime.

However, tests are executed one by one. Take a shortcut, and use a static
global for storing that state. This turns the test_context into a
singleton. To ensure it is not confused between multiple TESTs, add
asserts to verify its identity.

Following patches will add tests for notification callbacks. These will
be using the carried state.

[Pekka: add serialization checks, rename the global, rewrite commit message.]
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Jon A. Cruz <jonc@osg.samsung.com>