Memory leak fix: release source map info after processed and minor optimizations

[scotthovestadt]

Summary

Previously, sourceMaps:

• Was always populated with a call to getSourceMapInfo and set to an object, even when it was guaranteed to be empty.
• Was never released from memory.

It's companion, coverage, was released from memory but unfortunately using the delete operator, which is significantly slower than = undefined even with modern V8 versions:
https://jsperf.com/delete-vs-undefined-vs-null/87

I've resolved the leak and minor performance issues along with a minor change to formatTestResults that is slightly more efficient for the same result.

Test plan

• All tests pass.
• Memory and performance characteristics verified.

[codecov-io]

Codecov Report

Merging #8234 into master will decrease coverage by 0.03%.
The diff coverage is 31.81%.

@@            Coverage Diff             @@
##           master    #8234      +/-   ##
==========================================
- Coverage   62.33%   62.29%   -0.04%     
==========================================
  Files         265      265              
  Lines       10553    10554       +1     
  Branches     2565     2569       +4     
==========================================
- Hits         6578     6575       -3     
- Misses       3387     3391       +4     
  Partials      588      588

Impacted Files	Coverage Δ
packages/jest-runner/src/runTest.ts	2.43% <0%> (-0.1%)	⬇️
packages/jest-reporters/src/coverage_reporter.ts	59.54% <0%> (-0.46%)	⬇️
packages/jest-core/src/ReporterDispatcher.ts	83.33% <100%> (+2.08%)	⬆️
packages/jest-test-result/src/formatTestResults.ts	55.55% <50%> (-9.67%)	⬇️

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[github-actions]

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