Add --coverage-lcov and --branch-coverage options

[liamappelbe]

• Add a --coverage-lcov option to dart test, which takes a filename. This option works like --coverage, except that instead of generating multiple JSON reports, it generates a single LCOV report.
  • Aside from plumbing, the main change is in pkgs/test_core/lib/src/runner/coverage.dart's coverage collection functions, and how pkgs/test_core/lib/src/runner/engine.dart invokes them.
  • In LCOV mode, writeCoverage is still called, but doesn't write the JSON file. Instead it returns the generated coverage report. The Engine merges all these coverage reports into _allCoverageData, and once all the tests are done it calls writeCoverageLcov to finalize the report and write it to the lcov file.
• Add a --branch-coverage flag that is plumbed through to collect's branchCoverage param.
• Update the README to mention the --coverage-lcov workflow.
• Set collect's scopedOutput param to the current package name. This front load the filtering of the coverage report, improving performance by removing unnecessary information as early as possible.
• The test util runPub was using the deprecated standalone pub tool. This didn't cause problems because it was dead code. But I had to use it in the new tests, so I updated it to use dart pub.

Fixes #2511

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[liamappelbe]

The remaining test failures don't seem related to this PR