tests: Add span streaming tests (13) (#5620)

### Description
Span first on master is finally in a state where we can add meaningful
tests.

Longer term, we obviously want to run the whole existing test suite on
span first, but this will have to do for now.

These were written entirely by hand, so no need to go looking for
nonsensical mocks etc. (hopefully 😆).

#### Issues
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* resolves: #1234
* resolves: LIN-1234
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#### Reminders
- Please add tests to validate your changes, and lint your code using
`tox -e linters`.
- Add GH Issue ID _&_ Linear ID (if applicable)
- PR title should use [conventional
commit](https://develop.sentry.dev/engineering-practices/commit-messages/#type)
style (`feat:`, `fix:`, `ref:`, `meta:`)
- For external contributors:
[CONTRIBUTING.md](https://github.com/getsentry/sentry-python/blob/master/CONTRIBUTING.md),
[Sentry SDK development docs](https://develop.sentry.dev/sdk/), [Discord
community](https://discord.gg/Ww9hbqr)

Author: sentrivana