Fix tests exit race condition

[ijjk]

This ensures we don't allow a following exit condition to override a previous as async work is done before the process.exit actually occurs so a follow-up one could override the first giving a false negative.

Closes NEXT-1911

[ijjk]

Tests Passed

[ijjk]

Stats from current PR

Default Build

General

	vercel/next.js canary	ijjk/next.js fix/tests-exit-race	Change
buildDuration	12.9s	12.7s	N/A
buildDurationCached	7.2s	6.1s	N/A
nodeModulesSize	200 MB	200 MB	✓
nextStartRea..uration (ms)	426ms	426ms	✓

Client Bundles (main, webpack)

	vercel/next.js canary	ijjk/next.js fix/tests-exit-race	Change
193.HASH.js gzip	181 B	182 B	N/A
3f784ff6-HASH.js gzip	53.4 kB	53.4 kB	✓
433-HASH.js gzip	28.9 kB	28.9 kB	N/A
framework-HASH.js gzip	45.2 kB	45.2 kB	✓
main-app-HASH.js gzip	239 B	242 B	N/A
main-HASH.js gzip	31.8 kB	31.8 kB	N/A
webpack-HASH.js gzip	1.7 kB	1.7 kB	N/A
Overall change	98.6 kB	98.6 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	ijjk/next.js fix/tests-exit-race	Change
polyfills-HASH.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	ijjk/next.js fix/tests-exit-race	Change
_app-HASH.js gzip	194 B	195 B	N/A
_error-HASH.js gzip	183 B	181 B	N/A
amp-HASH.js gzip	504 B	502 B	N/A
css-HASH.js gzip	321 B	321 B	✓
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edge-ssr-HASH.js gzip	255 B	253 B	N/A
head-HASH.js gzip	350 B	349 B	N/A
hooks-HASH.js gzip	369 B	369 B	✓
image-HASH.js gzip	4.28 kB	4.28 kB	N/A
index-HASH.js gzip	255 B	256 B	N/A
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routerDirect..HASH.js gzip	312 B	311 B	N/A
script-HASH.js gzip	385 B	383 B	N/A
withRouter-HASH.js gzip	307 B	308 B	N/A
1afbb74e6ecf..834.css gzip	106 B	106 B	✓
Overall change	3.4 kB	3.4 kB	✓

Client Build Manifests

	vercel/next.js canary	ijjk/next.js fix/tests-exit-race	Change
_buildManifest.js gzip	483 B	484 B	N/A
Overall change	0 B	0 B	✓

Rendered Page Sizes

	vercel/next.js canary	ijjk/next.js fix/tests-exit-race	Change
index.html gzip	528 B	528 B	✓
link.html gzip	541 B	540 B	N/A
withRouter.html gzip	524 B	522 B	N/A
Overall change	528 B	528 B	✓

Edge SSR bundle Size

	vercel/next.js canary	ijjk/next.js fix/tests-exit-race	Change
edge-ssr.js gzip	93.9 kB	93.9 kB	N/A
page.js gzip	148 kB	148 kB	N/A
Overall change	0 B	0 B	✓

Middleware size

	vercel/next.js canary	ijjk/next.js fix/tests-exit-race	Change
middleware-b..fest.js gzip	624 B	626 B	N/A
middleware-r..fest.js gzip	151 B	151 B	✓
middleware.js gzip	37.5 kB	37.5 kB	N/A
edge-runtime..pack.js gzip	1.92 kB	1.92 kB	✓
Overall change	2.07 kB	2.07 kB	✓

Next Runtimes

	vercel/next.js canary	ijjk/next.js fix/tests-exit-race	Change
app-page-exp...dev.js gzip	169 kB	169 kB	✓
app-page-exp..prod.js gzip	95.4 kB	95.4 kB	✓
app-page-tur..prod.js gzip	96 kB	96 kB	✓
app-page-tur..prod.js gzip	90.6 kB	90.6 kB	✓
app-page.run...dev.js gzip	142 kB	142 kB	✓
app-page.run..prod.js gzip	90 kB	90 kB	✓
app-route-ex...dev.js gzip	24.2 kB	24.2 kB	✓
app-route-ex..prod.js gzip	16.8 kB	16.8 kB	✓
app-route-tu..prod.js gzip	16.8 kB	16.8 kB	✓
app-route-tu..prod.js gzip	16.4 kB	16.4 kB	✓
app-route.ru...dev.js gzip	23.6 kB	23.6 kB	✓
app-route.ru..prod.js gzip	16.4 kB	16.4 kB	✓
pages-api-tu..prod.js gzip	9.39 kB	9.39 kB	✓
pages-api.ru...dev.js gzip	9.67 kB	9.67 kB	✓
pages-api.ru..prod.js gzip	9.39 kB	9.39 kB	✓
pages-turbo...prod.js gzip	22 kB	22 kB	✓
pages.runtim...dev.js gzip	22.6 kB	22.6 kB	✓
pages.runtim..prod.js gzip	22 kB	22 kB	✓
server.runti..prod.js gzip	49.6 kB	49.6 kB	✓
Overall change	942 kB	942 kB	✓

Commit: cbefcc6