tst_WebContext_sessionCookies causes the test to abort in single process

[Chris Coulson]

This test hits a qFatal() in single-process:

PASS : qml-api-single-process-test::WebContext_sessionCookies::initTestCase()
QWARN : qml-api-single-process-test::WebContext_sessionCookies::test_WebContext_sessionCookies_ephemeral() OxideQQuickWebView: context is read-only in single process mode. The webview will automatically use the application-wide default WebContext
[0402/164821:ERROR:layer_tree_host_impl.cc(2205)] Forcing zero-copy tile initialization as worker context is missing
[0402/164821:ERROR:layer_tree_host_impl.cc(2205)] Forcing zero-copy tile initialization as worker context is missing
127.0.0.1 - - [02/Apr/2016 16:48:21] "GET /tst_WebContext_sessionCookies.py HTTP/1.1" 200 -
QWARN : qml-api-single-process-test::WebContext_sessionCookies::test_WebContext_sessionCookies_ephemeral() OxideQQuickWebView: context is read-only in single process mode. The webview will automatically use the application-wide default WebContext
QFATAL : qml-api-single-process-test::WebContext_sessionCookies::test_WebContext_sessionCookies_ephemeral() OxideQQuickWebContext::defaultContext: The default context has been deleted. This can happen in single process mode when the application creates and manages the default context
FAIL! : qml-api-single-process-test::WebContext_sessionCookies::test_WebContext_sessionCookies_ephemeral() Received a fatal error.
   Loc: [Unknown file(0)]

This fatal error basically means that the test deleted the default context and then tried to use it again. This is fatal because you can only have one context in single process.