test(utils): Remove usage of testOnlyIfNodeVersionAtLeast(8)

[AbhiPrasad]

We do not support Node 6 anymore, so we don't need to guard against
running on Node 6 in the tests.

Resolves https://getsentry.atlassian.net/browse/WEB-736

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	18.7 KB (-7.13% 🔽)
@sentry/browser - ES5 CDN Bundle (minified)	58.39 KB (-9.64% 🔽)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	17.42 KB (-7.66% 🔽)
@sentry/browser - ES6 CDN Bundle (minified)	52.55 KB (-9.35% 🔽)
@sentry/browser - Webpack (gzipped + minified)	19.2 KB (-17.37% 🔽)
@sentry/browser - Webpack (minified)	62.09 KB (-24.01% 🔽)
@sentry/react - Webpack (gzipped + minified)	19.22 KB (-17.41% 🔽)
@sentry/nextjs Client - Webpack (gzipped + minified)	42.89 KB (-10.75% 🔽)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	24.5 KB (-6.04% 🔽)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	22.97 KB (-6.19% 🔽)