Add JobGet + JobGetTx methods to Client, use within tests

[bgentry]

This adds some basic APIs for fetching a single job from the database. I updated all the tests in client_test.go to use it, except for some lower level ones that were using dbsqlc types and didn't seem worth converting.

[bgentry]

Lol, messed up my commits, fixing.

[brandur]

Looks great!

Should we cut a new release after merging this one? We may not ship anything else interesting for another week or two.

[brandur]

Hah, very unlikely, but small chance this fails locally because our test DB sequences don't get reset between runs. Probably will never happen, but I guess zero might be a slighter safer alternative.

[bgentry]

Should only be an issue if a row actually exists with this ID, right? And we either truncate after each test or use a txn that gets rolled back.

[bgentry]

Should we cut a new release after merging this one? We may not ship anything else interesting for another week or two.

I don't think so quite yet. I will have another new API up soon (something like JobRetryNow to immediately schedule any job that's not currently running) so maybe after that we can cut a new one. Plus I'd like to test both of these further in my UI repo just to be sure.