Use test's `.meta/index.json` for test registration

[chfast]

EEST's fixtures contains the file .meta/index.json with the mapping of the test name to the JSON test file.

If a test tool (e.g. evmone-statetest) is pointed at a directory and this directory contains this file use the information there to register tests by their names instead of by file names found in the directory.

A test file may contain multiple tests.

[pdobacz]

note: consider using consume direct (or ctest) in conjunction with .meta/index.json in order to avoid parsing and working with the meta file ourselves (statetest & blockchaintest would then become minimal tools able to run just a single .json file)