Rework PET job in `test.yml` workflow

[rockbmb]

This repository contains a testing workflow in ./.github/workflows/test.yml, which includes the Polkadot Ecosystem Test suite (presently disabled).

The PET job in this workflow is currently as follows:

runtimes/.github/workflows/test.yml

Lines 334 to 339 in c1a537c

	- name: Running ecosystem tests
	uses: nick-fields/retry@v3
	with:
	timeout_minutes: 90
	max_attempts: 3
	command: cd ecosystem-tests && yarn test

This is not ideal, because with PET having recently gone beyond 100+ individual test suites, a single failure, which can happen for various reasons (RPC timeouts are frequent), will cause the above step to be retried from the beginning.

To mitigate this, each individual test suite should run independently of the others, and the timeout/retry mechanisms should apply individually.

[rockbmb]

I offer to work on this, as I contribute to PET frequently.

[rockbmb]

@bkchr regarding my comment last week, I am testing some changes in PET CI that will be helpful here.

If they work as expected, this issue will be a relatively straightforward corollary.