tests: spi_loopback: Clean up file defines

Following the boy scout rule, since I am using this test a lot lately
and going to improve it, the first step will be to clean up the
structure of the file. The following changes are made:

- Remove unused #includes
- Condense some #if #else macro defines or make into single line, to
  simplify the readability of the preprocessor code
- Move definitions around so that related things are all next to
  each other, instead of randomly scattered around the file
- Create section header comments for broadly related things in the file
  to improve developer navigation experience
- Introduce macro for copy pasted print buf size calculation
- A few minor comment edits

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>