Use c8's --merge-async option when generating coverage from the root

[Abe27342]

Description

This should resolve the intermittent OOM issues we've been seeing on CI. The cause of them appears was this issue, as by default c8 attempts to merge coverage reports by loading them all into memory at the same time. Our root c8 config includes many files, so this can trigger OOM issues.

I've updated the repo to c8@8.0.1, since we needed to bump the c8 dependency anyway to use the merge-async option and the only breaking change between 7 and 8 was dropping node 10 support.

[alexvy86]

Tangent to this PR: I question a bit if we should have c8 as a devDependency on every package? The way we do test coverage is by running with c8 from the root package.json and just running test:mocha or similar on each package. Some packages do have a test:coverage script that uses c8 but AFAIK we don't leverage them?

[msfluid-bot]

⯅ @fluid-example/bundle-size-tests: +253 Bytes

Metric Name	Baseline Size	Compare Size	Size Diff
aqueduct.js	734.91 KB	734.96 KB	⯅ +44 Bytes
connectionState.js	680 Bytes	680 Bytes	■ No change
containerRuntime.js	533.55 KB	533.57 KB	⯅ +22 Bytes
loader.js	178.56 KB	178.58 KB	⯅ +22 Bytes
map.js	552.1 KB	552.13 KB	⯅ +33 Bytes
matrix.js	651.61 KB	651.64 KB	⯅ +33 Bytes
odspDriver.js	99.31 KB	99.34 KB	⯅ +22 Bytes
odspPrefetchSnapshot.js	52.39 KB	52.41 KB	⯅ +11 Bytes
sharedString.js	669.3 KB	669.33 KB	⯅ +33 Bytes
sharedTree.js	780.39 KB	780.41 KB	⯅ +22 Bytes
Total Size	4.27 MB	4.27 MB	⯅ +253 Bytes

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Baseline commit: 837c4ba

Generated by 🚫 dangerJS against f5ceeac

[Abe27342]

Tangent to this PR: I question a bit if we should have c8 as a devDependency on every package? The way we do test coverage is by running with c8 from the root package.json and just running test:mocha or similar on each package. Some packages do have a test:coverage script that uses c8 but AFAIK we don't leverage them?

yeah. it seems weird to me too.