Reset context between test functions

[thymikee]

Summary

Currently the this context we're passing in before/after (although here this is undefined, and I didn't even try to investigate why) and it/test functions is preserved across consequent test runs, which makes it confusing to use (see #3505).

As a workaround I propose passing empty object as a context for each function. This way we can still use this in test but it will be unique for each test function.

Fixes #3505.

Test plan

I'll add an integration test later, after we agree this is the right solution.

[codecov-io]

Codecov Report

Merging #3506 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master    #3506   +/-   ##
=======================================
  Coverage   62.28%   62.28%           
=======================================
  Files         181      181           
  Lines        6700     6700           
  Branches        5        5           
=======================================
  Hits         4173     4173           
  Misses       2525     2525           
  Partials        2        2

Impacted Files	Coverage Δ
packages/jest-jasmine2/src/jasmine-async.js	0% <0%> (ø)	⬆️

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[github-actions]

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