[SYNTH-14523] Only auto-discover with default glob when nothing given

What and why?

This PR fixes a regression introduced by #1214:

The user has tests A, B and C defined in configuration files, with specific overrides that are important ⚠️.
The user relies on the auto-discovery:
- They are omitting the --files argument, thus defaulting to {,!(node_modules)/**/}*.synthetics.json
Just to test something locally, they want to use --public-id A to "focus" on test A and only run that test.

Before #1214

Passing a --public-id would disable the auto-discover → the specific overrides would not be loaded → the test wouldn't behave as expected ❌

After #1214

Passing a --public-id now keeps the auto-discover → config files containing the specific overrides are discovered → test A is run and configured properly ✅

The problem

Sadly, this came with a big performance impact when running datadog-ci from the user's home directory with just a --public-id parameter, because the CLI is traversing the whole disk (from the home directory).

How?

Go back to the previous behavior, unless --files is set explicitly:

Only auto-discover with the default glob if the user doesn't give any clue what to run.
- "No clue" means no publicIds, nor files, nor testSearchQuery (nor suites array if using the library)
If they give publicIds only, we shouldn't auto-discover for performance reasons. (old behavior)
If they give publicIds and files, then only the given publicIds are run and the test config files are applied on them. (keeping the intended fix from #1214)

Review checklist

[x] Feature or bugfix MUST have appropriate tests (unit, integration)

DataDog / datadog-ci