ref(nextjs): Small improvements to config tests

[lobsterkatie]

In the process of reviewing #3922, I realized that there was some cleanup work I could do to make that PR work better. Specifically:

• withSentryConfig had an incorrect type for one of the arguments to the userNextConfig function.
• The mock for that same argument was missing its outer layer, and therefore wasn't really what it said it was.
• In real life, the config property of buildContext is equal to the combination of next's default config and the user-provided config. The mock wasn't reflecting this.
• That PR was having to compensate for the fact that the mocks for build context were static rather than dynamic, even though those test cases especially illustrate the ways in which the value of the mock needs to change depending on other values in the test. Where necessary, the build context is now calculated as part of the test.

[github-actions]

size-limit report

Path	Size
@sentry/browser - CDN Bundle (gzipped)	21.6 KB (0%)
@sentry/browser - Webpack	22.61 KB (0%)
@sentry/react - Webpack	22.64 KB (0%)
@sentry/browser + @sentry/tracing - CDN Bundle (gzipped)	29.03 KB (0%)

[kamilogorek]

Just a small simplification but feel free to ignore it