Fix memory errors in sync unit tests

Fix memory errors in sync unit tests

The commit r133754 introduced some errors.  That change made the tests
delete the session a bit more frequently, while some of the tests
maintained more pointers to its internal structures.

This change fixes all the affected tests and adds a member function to
the test framework class to try to prevent this from happening in the
future.

BUG=122033, 123270
TEST=

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=133776

[rlarocque, 2012-04-24 21:49:17]

This should fix the build bot flakiness.  This passes valgrind locally on my
machine, while an unmodified ToT build does not.

[tim (not reviewing), 2012-04-24 21:51:25]

LGTM