support scala 3 build for util-test

[felixbr]

This PR builds on https://github.com/twitter/util/pull/304 so that should be merged before this one imo.

There is no Scala 3 compatible version for "org.scalatestplus" %% "mockito-1-10", so I had to use "org.scalatestplus" %% "mockito-3-4" % "3.2.9.0". This means I upgraded Scalatest to 3.2.9 and Mockito to 3.4.x. This doesn't really break any code in util-test but consumers of util-test (e.g. finagle or twitter-server) might also have to upgrade those test dependencies.

Since Scala 3 requires Scalatest 3.2.x and Mockito 1.10.x is ancient at this point, I find this acceptable.

The second point is that I couldn't find a direct replacement for org.mockito.exceptions.Reporter from Mockito 1.10.x. Since it was only used to throw a predefined exception, I inlined the error message. I hope this is acceptable.

[codecov-commenter]

Codecov Report

Merging #306 (fa80c54) into develop (9f15cce) will increase coverage by 0.01%. The diff coverage is 0.00%.

@@             Coverage Diff             @@
##           develop     #306      +/-   ##
===========================================
+ Coverage    49.29%   49.30%   +0.01%     
===========================================
  Files          322      322              
  Lines        18523    18522       -1     
  Branches      1055     1063       +8     
===========================================
+ Hits          9131     9133       +2     
+ Misses        9392     9389       -3     

Impacted Files	Coverage Δ
...ala/com/twitter/util/testing/ArgumentCapture.scala	5.91% <0.00%> (+<0.01%)	:arrow_up:
util-core/src/main/scala/com/twitter/io/Buf.scala	93.27% <0.00%> (ø)
...core/src/main/scala/com/twitter/util/Promise.scala	79.84% <0.00%> (+0.38%)	:arrow_up:
...ore/src/main/scala/com/twitter/util/Duration.scala	85.39% <0.00%> (+0.56%)	:arrow_up:

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[felixbr]

CI failed because sbt-scoverage was updated to 2.0.0 here.

Once develop is green again, I can rebase this PR once more, which should fix it.

[pjfanning]

CI failed because sbt-scoverage was updated to 2.0.0 here.

Once develop is green again, I can rebase this PR once more, which should fix it.

I created https://github.com/twitter/util/pull/309

[Edit] and I only just spotted https://github.com/twitter/util/pull/307

[bryce-anderson]

@pjfanning, the scoverage thing should be fixed now.

[felixbr]

I've rebased this onto develop.