test: repair web3.js getBlocks tests

[steveluscher]

Problem

These tests were never correct, because getBlocks is supposed to return the blocks between startSlot and endSlot inclusive.

I think they were accidentally working before, because of a bug in the validator that has since been fixed in #23853.

Summary of Changes

• Wait for there to be at least 2 blocks, so that the tests have something to exercise.
• Fix the mock data to actually return the blocks between startSlot and endSlot inclusive.
• Make the test resilient to pruning by setting startSlot to whatever firstAvailableBlock returns (this will make the web3.js tests less flaky 🎉)

[codecov]

Codecov Report

Merging #24813 (0e7aac5) into master (8ba003a) will decrease coverage by 11.8%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master   #24813       +/-   ##
===========================================
- Coverage    81.8%    70.0%    -11.9%     
===========================================
  Files         632       38      -594     
  Lines      167499     2303   -165196     
  Branches      322      325        +3     
===========================================
- Hits       137169     1614   -135555     
+ Misses      30217      573    -29644     
- Partials      113      116        +3     

[CriesofCarrots]

Much better, thank you!