vmh: regions_mm: Extend vmh tests

[softwarecki]

Fixed type returned by sys_run_boot_tests function. Fixed code formatting vmh test suite to be consistent with the principles adopted in the project. Extended verify_memory_content function to use different tags at the beginning, middle and end of the buffer. A value of the tags depends on the buffer address. Write of tags has been separated from verifying their correctness. This makes it possible to check whether the contents of the buffer haven't been accidentally modified. Due to the increase of the tag size, it was necessary to increase the minimum buffer size in test functions.

Added a test that allocates all available buffers. It also operates on buffers larger than the size of a memory page.