Add test coverage reporting

[outofambit]

closes #3385

• adds a third build of p5.js that includes instrumentation to measure code coverage.
• general summary of test coverage is printed out after running tests
• detailed html page of test coverage is generated at coverage/index.html

[meiamsome]

Any errors in these will not be caught properly and might be problematic - I would probably consider doing something like

          const { stats, coverage }  = await new Promise(resolve => eventHandler1.on('mocha:end', resolve));
          if (stats.failures) {
            throw new Error(stats);
          }
          if (coverage) {
            await fs.mkdir('./.nyc_output/', { recursive: true });
            await fs.writeFile(
              './.nyc_output/out.json',
              JSON.stringify(coverage)
            );
          }

[outofambit]

how does this look now? i tried the above but had some issues so i tried to find something between that and the current structure.