A common error in all.py is that two different tests are assigned to the
same key value in profile.test_list. This change prevents a key from
being reassigned to a new value on accident, but provides a mechanism to
allow reassignment, using a context manager. This allows tests like the
image_load_store tests in quick.py to be overwritten, but requires the
author to mark that they know that they are overwriting tests, and that
it's intentional.

The error message will compare the tests by command, if the two tests
have different commands then the commands will be printed for easy
visual comparison. This helps to identify whether a test is being added
twice (in a loop for example) or if two different tests are being
assigned the same name.

This also adds some tests for TestDict.

v2: - remove some rebase artifacts, this makes the code a little simpler
      and a little cleaner
v3: - rebase on master, includes grouptools separator changes
v4: - Use a incremented value instead of True false for the context
      manager, this allows the allow_reassignment contextmanager to be
      nested.  (Ilia)
v5: - Add comparison methods to the Test class
    - Use said comparisons to make the error reporting more verbose

Signed-off-by: Dylan Baker <dylanx.c.baker@intel.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu> (v4)