syz-cluster/pkg/db: fix bunch of bugs in emulator management

There is a bunch of bugs now:
1. The emulator binary is killed when the first test finished,
before subsequent tests start.
2. The child emulator binary (the actual emulator "emulator_main") is leaked.
These subprocesses are never killed and live past tests.
(that's why we get away with the first problem)
3. Errors are not handled well if emulator setup fails.
We leave spannerHost empty and subsequent tests ignore the original error
(since only the first test executes setupSpannerOnce).
4. NewTransientDB duplicates a bunch of work that needs to happen only once
(in particular os.Setenv("SPANNER_EMULATOR_HOST")).

Fix all of that.

Support spanner emulator distributed as part of google-cloud-sdk while we are here,
so that tests can be run locally.

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