test: fix flakey test

[alexghr]

This PR attempts to fix the flakey epochs_empty_blocks test. There were a couple of problems with this test resulting from #13428:

• the test assumes it will run for N consecutive epochs
• but the test also waits for certain blocks to become proven/finalised/historical
• in feat: Support for L1 reorgs #13428 the meaning of 'finalised' was changed. This caused the test to skip entire epochs (e.g. entire epochs would be proven in the background while it waited for historical blocks)
• this PR replicates the 'finalised' logic from the archiver so that the epoch counter updates correctly