Rename spec_kst_to_all to build-tests and better document process of testing

[Mingun]

In the #108 (comment) @generalmimon was not against to removing existing build-tests script which is not very useful. On the other hand, spec_kst_to_all script which is actually useful:

• named unclear
• contains underscores which are slightly inconvenient to enter (requires pressing SHIFT + - in QWERTY layout)

This PR solves both problems by removing the old build-tests script and renaming spec_kst_to_all script to build-tests. It also updates documentation, because I find it is difficult to have documentation of how to run tests far away from tests itself in other repository (https://github.com/kaitai-io/kaitai_struct_doc/blob/c87ae4cb447abfb043bcbade1c8d601037469142/kst.adoc).

I also described directories that was not described to have the full picture in the Readme.

This PR should be merged together with kaitai-io/kaitai_struct_doc#55 to update mentioned kst.adoc.

[Mingun]

@generalmimon , is it possible to merge this PR? It greatly simplifies developing process, because you should not remember to make sbt publishLocal after building compiler.