chore(naming): Rename browser manual test folder for clarity

[lobsterkatie]

There are currently three folders in the repo called manual, as part of the testing suites of raven-node, @sentry/node, and @sentry/browser, respectively:

https://github.com/getsentry/sentry-javascript/tree/5.12.3/packages/raven-node/test/manual
https://github.com/getsentry/sentry-javascript/tree/5.12.3/packages/node/test/manual
https://github.com/getsentry/sentry-javascript/tree/5.12.3/packages/browser/test/manual

For the first two folders, that name makes sense, as each houses an express app designed to let you manually trigger various scenarios and see how the SDK reacts. For the third, however, it doesn't, as that folder contains code designed to test @sentry/browser as a package (rather than just as a bundle). This PR renames the folder such that its name is more in line with its contents.

[getsentry-bot]

	Warnings
⚠️	Please add a changelog entry for your changes.

	Messages
📖	✅ TSLint passed
📖	@sentry/browser bundle gzip'ed minified size: (ES5: 16.7168 kB) (ES6: 15.7246 kB)

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