Attest: Remove unnecessary asym/sym test cases

- Remove buffer_too_small_test(): the MINIMAL_TOKEN_SIZE
  macros need to be maintained and are not entirely reliable.
  This negative test case is now performed based on the queried
  expected size of the token rather than preliminary assumptions.
- Remove minimal_get_size_test() - it does not have much added value,
  the token is validated thoroughly in test case 1001,
- remove TOO_SMALL_TOKEN_BUFFER (unused).

Change-Id: I7b14f0c35bf62437ca94dadb5d7ed91a4f745c9f
Signed-off-by: David Vincze <david.vincze@arm.com>
8 files changed