Fix flakey preload test

[ijjk]

This ensures we only check for the related preload for the test so that any other preloads in the viewport don't cause a false positive.

Fixes: https://github.com/vercel/next.js/runs/4103245093?check_suite_focus=true
x-ref: #30963 (comment)

[ijjk]

Stats from current PR

Default Build (Decrease detected ✓)

General Overall decrease ✓

	vercel/next.js canary	ijjk/next.js fix/flakey-preload-test	Change
buildDuration	25.1s	25.5s	⚠️ +417ms
buildDurationCached	5s	5s	-17ms
nodeModulesSize	332 MB	332 MB	-13 B

Page Load Tests Overall decrease ⚠️

	vercel/next.js canary	ijjk/next.js fix/flakey-preload-test	Change
/ failed reqs	0	0	✓
/ total time (seconds)	4.128	4.196	⚠️ +0.07
/ avg req/sec	605.64	595.81	⚠️ -9.83
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	2.206	2.207	0
/error-in-render avg req/sec	1133.47	1133	⚠️ -0.47

Client Bundles (main, webpack, commons)

	vercel/next.js canary	ijjk/next.js fix/flakey-preload-test	Change
450.HASH.js gzip	179 B	179 B	✓
framework-HASH.js gzip	42.2 kB	42.2 kB	✓
main-HASH.js gzip	28 kB	28 kB	✓
webpack-HASH.js gzip	1.45 kB	1.45 kB	✓
Overall change	71.9 kB	71.9 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	ijjk/next.js fix/flakey-preload-test	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/flakey-preload-test	Change
_app-HASH.js gzip	1.23 kB	1.23 kB	✓
_error-HASH.js gzip	194 B	194 B	✓
amp-HASH.js gzip	312 B	312 B	✓
css-HASH.js gzip	327 B	327 B	✓
dynamic-HASH.js gzip	2.38 kB	2.38 kB	✓
head-HASH.js gzip	350 B	350 B	✓
hooks-HASH.js gzip	635 B	635 B	✓
image-HASH.js gzip	4.44 kB	4.44 kB	✓
index-HASH.js gzip	263 B	263 B	✓
link-HASH.js gzip	1.87 kB	1.87 kB	✓
routerDirect..HASH.js gzip	321 B	321 B	✓
script-HASH.js gzip	383 B	383 B	✓
withRouter-HASH.js gzip	318 B	318 B	✓
334f979574ae..6f4.css gzip	106 B	106 B	✓
Overall change	13.1 kB	13.1 kB	✓

Client Build Manifests

	vercel/next.js canary	ijjk/next.js fix/flakey-preload-test	Change
_buildManifest.js gzip	459 B	459 B	✓
Overall change	459 B	459 B	✓

Rendered Page Sizes

	vercel/next.js canary	ijjk/next.js fix/flakey-preload-test	Change
index.html gzip	523 B	523 B	✓
link.html gzip	535 B	535 B	✓
withRouter.html gzip	516 B	516 B	✓
Overall change	1.57 kB	1.57 kB	✓

---

Default Build with SWC (Increase detected ⚠️)

General Overall decrease ✓

	vercel/next.js canary	ijjk/next.js fix/flakey-preload-test	Change
buildDuration	27.7s	27.2s	-492ms
buildDurationCached	5.1s	5s	-124ms
nodeModulesSize	332 MB	332 MB	-13 B

Page Load Tests Overall increase ✓

	vercel/next.js canary	ijjk/next.js fix/flakey-preload-test	Change
/ failed reqs	0	0	✓
/ total time (seconds)	4.234	4.187	-0.05
/ avg req/sec	590.47	597.04	+6.57
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	2.241	2.213	-0.03
/error-in-render avg req/sec	1115.62	1129.47	+13.85

Client Bundles (main, webpack, commons)

	vercel/next.js canary	ijjk/next.js fix/flakey-preload-test	Change
450.HASH.js gzip	179 B	179 B	✓
framework-HASH.js gzip	42.3 kB	42.3 kB	✓
main-HASH.js gzip	28.2 kB	28.2 kB	✓
webpack-HASH.js gzip	1.43 kB	1.43 kB	✓
Overall change	72.1 kB	72.1 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	ijjk/next.js fix/flakey-preload-test	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/flakey-preload-test	Change
_app-HASH.js gzip	1.22 kB	1.22 kB	✓
_error-HASH.js gzip	180 B	180 B	✓
amp-HASH.js gzip	305 B	305 B	✓
css-HASH.js gzip	321 B	321 B	✓
dynamic-HASH.js gzip	2.38 kB	2.38 kB	✓
head-HASH.js gzip	342 B	342 B	✓
hooks-HASH.js gzip	622 B	622 B	✓
image-HASH.js gzip	4.46 kB	4.46 kB	✓
index-HASH.js gzip	256 B	256 B	✓
link-HASH.js gzip	1.91 kB	1.91 kB	✓
routerDirect..HASH.js gzip	314 B	314 B	✓
script-HASH.js gzip	375 B	375 B	✓
withRouter-HASH.js gzip	309 B	309 B	✓
334f979574ae..6f4.css gzip	106 B	106 B	✓
Overall change	13.1 kB	13.1 kB	✓

Client Build Manifests

	vercel/next.js canary	ijjk/next.js fix/flakey-preload-test	Change
_buildManifest.js gzip	460 B	460 B	✓
Overall change	460 B	460 B	✓

Rendered Page Sizes

	vercel/next.js canary	ijjk/next.js fix/flakey-preload-test	Change
index.html gzip	522 B	522 B	✓
link.html gzip	535 B	535 B	✓
withRouter.html gzip	516 B	516 B	✓
Overall change	1.57 kB	1.57 kB	✓

Commit: 8d30043