selftests/tpm2: Extend tests to cover partial reads

Three new tests added:
1. Send get random cmd, read header in 1st read, read the rest in second
   read - expect success
2. Send get random cmd, read only part of the response, send another
   get random command, read the response - expect success
3. Send get random cmd followed by another get random cmd, without
   reading the first response - expect the second cmd to fail with -EBUSY

Signed-off-by: Tadeusz Struk <tadeusz.struk@intel.com>
Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Tested-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Signed-off-by: James Morris <james.morris@microsoft.com>