Fix test failure in fresh checkout of repository

[douglance]

Description of Changes

Use relative import '../src' in serde.test.ts to match all other test files.

This is a consistency fix. The 'spacetimedb' import works in CI (build runs first, enabling package self-reference), but all other tests use '../src'.

API and ABI breaking changes

None

Expected complexity level and risk

1

[bfops]

Hey @douglance, thank you for taking the time to make a contribution!

To minimize noise in the repo, we generally try to avoid small preference-inspired PRs (unless they're specifically bringing us in line with our linter, style guide, etc.)

I hope that's understandable. Please let me know if I've missed something important about the motivation or impact here!

[douglance]

@bfops This fixes an issue where the tests fail unless the system has been built. It's not a preference thing.

I lost some time trying to figure out why this test was failing on a fresh pull down of the repo.