Run Tekton tests in parallel

[felipecruz91]

Currently the Tekton tests are executed sequentially.

Investigate and update the testing framework accordingly to support parallel execution.

Bear in mind:

• Test cases that share the same workspace could end up corrupting files.

• Define a safe number of parallel executions that won't cause resource exhaustion in the GitHub Action machine.

[michaelsauter]

Test cases that share the same workspace could end up corrupting files.

This should already be solved: workspaces sources are copied and placed into random folders.

Define a safe number of parallel executions that won't cause resource exhaustion in the GitHub Action machine.

AFAIK we can tune this when running go test, so just have to find a setting that works :)

[michaelsauter]

The main motivation for this was the long duration of the tests. This should no longer be the case since #722.