generate test easyconfig without checksums to check whether checksums.json is picked up in test_checksum_step

[boegel]

@mboisson Some cleanup for the tests in easybuilders#3749.

In general, it's better to avoid adding additional test easyconfigs if it can be avoided, since it complicates maintenance of the test suite, and may slow down the tests that iterate over all available test easyconfigs...

Also, I don't see the need to use checksums-test.json and copying that to checksums.json in the test, so I renamed test_ecs/t/toy/checksums-test.json to test_ecs/t/toy/checksums.json.
To avoid breaking the test_enforce_checksums, where checksums.json is not supposed to be found, I've wiped $EASYBUILD_ROBOT_PATHS in that test (since then test_ecs/t/toy is no longer consider by obtain_file while it tries to find checksums.json.