Implement RAM tests

Based on work of radioman @radiomanV (see branch vtable, commit aefc58c4 in https://gitlab.com/radiomanV/minipro).

Unlike the original work, I decided not to create a separate option for running RAM testing: no one needs to remember another option -- we already have one. Furthermore, the chip interface may have specific features that we can test using the existing mechanism.

See doc/test-ram.md for SRAM testing examples.

The firmware request for RAM testing for the TL866A in the original work looks strange, and I don't have such a programmer for testing, so I'm leaving the work of adding support for it to its owners.