This was hard. The problem was that the mock PKCS#11 module accepts only
a single PKCS#11 session at a time. The test normally creates one
session, closes it, creates another, and closes it. But sometimes the
first session doesn't get closed before the second is created, and the
test breaks. The problem is that the PKCS#11 session is created and
closed by the GTlsClientConnection. The test creates two, and unrefs the
first one before creating the second, but other refs are held by the
first connection's GTasks. These GTasks complete in idle callbacks, and
if one idle fails to run before we finish iterating the main loop, the
first GTlsClientConnection won't be destroyed and so the first PKCS#11
session won't be closed in time. Note there is no bug in the
implementation, and nothing is leaked; rather, it's just not destroyed
soon enough for the test.

Fix this by iterating the test's main context until the first connection
is destroyed.