Adding a CacheOnly integration test

[gauntface]

R: @jeffposnick

Just a simple test for CacheOnly.

[workbox-pr-bot]

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View Table

File	Before	After	Change	GZipped
packages/workbox-background-sync/build/workbox-background-sync.prod.js	3.17 KB	3.17 KB	0%	1.34 KB
packages/workbox-broadcast-cache-update/build/workbox-broadcast-cache-update.prod.js	1.05 KB	1.05 KB	0%	565 B
packages/workbox-build/build/_types.js	41 B	41 B	0%	61 B
packages/workbox-build/build/index.js	2.52 KB	2.52 KB	0%	1.06 KB
packages/workbox-cache-expiration/build/workbox-cache-expiration.prod.js	3.25 KB	3.25 KB	0%	1.20 KB
packages/workbox-cacheable-response/build/workbox-cacheable-response.prod.js	593 B	593 B	0%	343 B
packages/workbox-cli/build/app.js	4.57 KB	4.57 KB	0%	1.65 KB
packages/workbox-cli/build/bin.js	2.53 KB	2.53 KB	0%	1.07 KB
packages/workbox-core/build/workbox-core.prod.js	6.67 KB	6.67 KB	0%	2.61 KB
packages/workbox-google-analytics/build/workbox-google-analytics.prod.js	1.94 KB	1.94 KB	0%	985 B
packages/workbox-precaching/build/workbox-precaching.prod.js	4.86 KB	4.86 KB	0%	1.89 KB
packages/workbox-range-requests/build/workbox-range-requests.prod.js	1.66 KB	1.66 KB	0%	817 B
packages/workbox-routing/build/workbox-routing.prod.js	2.74 KB	2.74 KB	0%	1.23 KB
packages/workbox-strategies/build/workbox-strategies.prod.js	3.22 KB	3.22 KB	0%	1.00 KB
packages/workbox-sw/build/workbox-sw.js	1.46 KB	1.46 KB	0%	792 B
packages/workbox-webpack-plugin/build/index.js	7.29 KB	7.29 KB	0%	2.45 KB

Workbox Aggregate Size Plugin

☠️ WARNING ☠️

We are using 152% of our max size budget.

Total Size: 22.25KB
Percentage of Size Used: 152%

Gzipped: 8.85KB

[coveralls]

Coverage remained the same at 84.77% when pulling 91cbb6a on cacheonly-integration into 21c8338 on v3.