mtest: Fix test selection implementation

Omitting test()'s `suite` kwarg used to default to `test.suite==['']`
because of how stringlistify() works. That had inconsistent behavior
with `suite: []` which makes `test.suite==[]`. That changed when porting
to typed_kwarg (#8855), which made the default `test.suite==[]`.

Change `suite: []`, and missing kwarg, back to `test.suite==['']` which
was the original intention. It has impact on `meson test --suite`
selection, an empty `test.suite` list means that test is never going to
be selected when `--suite` is used, even when we want all tests from a
given subproject.

It was also unclear what `--suite foo` means. Is `foo` a suite or a
project name? It can now be either, it selects all tests from project
`foo` as well as all tests from any project in suite `foo`.

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