kselftest: convert get_size to use stricter TAP13 format

1. Add the TAP13 header
2. remove variable data from the test description line
3. move the plan count to the end of the file, for consistency with
other kselftests
4. convert memory data from diagnostic (comment) format, to a YAML block

Signed-off-by: Tim Bird <tim.bird@sony.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>