Change the way blocks are mined during testing

[pcarranzav]

Since #517, our hardhat local network mines blocks in two ways:

• Automatically when any transaction is sent
• Every 30 seconds

I suspect this is breaking our unit tests for a few reasons:

• CI has started failing quite frequently since that commit
• Several of the failed tests look like timing issues
• I manually timed running tests and noticed errors tend to happen close to 30-second boundaries (though this was a bit of a handwavy measurement).

This PR changes things in two ways:

• For the default local network, disable automining, so transactions will not trigger a mined block, and instead they will only be mined every 30 seconds (@fordN could you confirm if this is the desired behavior?)
• For the unit tests, enable automining and disable the mining interval.

I've run yarn test several times in this branch and haven't seen it fail so far (whereas it often fails if I run it from the main dev branch).

An additional small change: we allow passing file paths to yarn test so that we can optionally run only specific testfiles.

[codecov]

Codecov Report

Merging #554 (5faa832) into dev (3538e00) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev     #554   +/-   ##
=======================================
  Coverage   90.58%   90.58%           
=======================================
  Files          37       37           
  Lines        1795     1795           
  Branches      293      293           
=======================================
  Hits         1626     1626           
  Misses        169      169           

Flag	Coverage Δ
unittests	90.58% <ø> (ø)

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[abarmat]

Good find! I'm good with it, only thing would be to check if Ford needs automine by default

[fordN]

Works for me! The current setup with both automining and timed mining was somewhat arbitrary.