Support for test-only layers in testsuites.

### Description

Support for including (merging) a layer fragment for testing would be useful.

### Use case/motivation

I needed to write a test that injects something into layers, so that NB implementation sees testing data through FileUtil.getConfigFile and lookup. In simple cases, it is sufficient to provide META-INF/generated-layer.xml, which is picked by the module system automatically from classpath. But in my case, the testsuite uses layer-generating annotations that themselves make such file, effectively overwriting the custom one.

There's some support in Java's `SourceUtilsTestUtil` in `java.source.base` - but I think the support should be promoted, most probably to `openide.filesystems` that contains various filesystem-related or generally to some `platform` cluster module that tests can depend on.

Alternatively, another layer-generating annotation can be created (e.g. in `openide.filesystems`, that would copy over resource content into the generated layer.xml

### Related issues

_No response_

### Are you willing to submit a pull request?

Yes

Start with SourceUtilsTestUtil in java.source.base and inspect the testsuite layer-generating annotations, along with how META-INF/generated-layer.xml is consumed by FileUtil.getConfigFile and lookup. Compare the proposed openide.filesystems or platform-cluster locations and define a test-only layer merge that does not get overwritten by generated layers.

Written by the indexing model from the issue text.