Avoid testing global InstrumentationSystem state

[kukushechkin]

Refactor tests to only use InstrumentationSystem global state bootstrapping where necessary.

Motivation:

Decoupling tests from the global state allows running them in parallel later on within Swift Testing.

Modifications:

• Use local tracer variable instead of calling InstrumentationSystem.bootstrapInternal()
• Add tests specifically for global withSpan()-like methods

Result:

Most of the tests no longer rely on the global InstrumentationSystem state. Test coverage is up slightly.

[ktoso]

Cool, lgtm 👍

[ktoso]

Technically this is "semver/none" not patch I guess, no real functionality touched at all