promqle2etest: Optional parallelization

[bwplotka]

We might want to revisit the Run abstraction. Doing t.Run +t.Parallel deep nested functions can be confusing for Run callers as:

• Every code that's put after Run will still be run before Run(!) execution.
• Its not possible to perform logic AFTER Run (knowing when Run succeeded) - this is useful when you would like to test more things after Run e.g. Metadata API

See GoogleCloudPlatform/prometheus#246