Improve unit test support for snapshots

[canepat]

We use some sample header/body/transaction snapshots for unit testing, which were generated without any specific automation some time ago and have a slightly custom format, in the sense they start at block 1'500'012 and contain just 2 blocks. See here for more details.

A better overall solution would be:

• introduce some subcommand in snapshots command-line utility to generate (short) sample snapshots
• use some other command-line utility (maybe new embed_binary?) to generate C++ code for sample snapshots
• regenerate sample snapshots without custom start point (i.e. add initial empty blocks)
• remove IndexBuilder::set_base_data_id as it becomes unnecessary

See also #2419 (comment)