I have a project containing both sources and tests and I want to separate main jars (code and sources) from test ones, uploading test artifacts with a dedicated artifact id (i.e. project name + '-tests').
To obtain this result I produced my tests jar with an appropriate classifier and added two filters to MavenDeployer to filter jars and change the artifact id for the pom containing the tests (based upon the classifier attribute value)

uploadArchives {
    //uploads archives to private maven-compatible repository
    repositories.mavenDeployer {
        repository (url: "${libs_releases_local}")  {
                authentication (userName: "${artifactory_user}", password: "${artifactory_pass}")
        }
        snapshotRepository (url: "${libs_snapshots_local}") {
                authentication (userName: "${artifactory_user}", password: "${artifactory_pass}")
        }
        addFilter('main') {artifact, file ->
                artifact.attributes['classifier'] != 'tests' //artifact whose clssifier attribute is NOT 'tests'
        }
        addFilter('tests') {artifact, file ->
                artifact.attributes['classifier'] == 'tests' //artifact whose clssifier attribute is 'tests'
        }
        pom('tests').artifactId = "${project.name}-tests"

This code produces unstable results... for instance sometimes I obtain this log

Uploading: org/myorg/Testproject/0.1/Testproject-0.1.jar to repository remote at ...
Transferring 2K from remote
Uploaded 2K
Uploading: org/myorg/Testproject/0.1/Testproject-0.1-sources.jar to repository remote at ...
Transferring 1K from remote
Uploaded 1K
Uploading: org/myorg/Testproject-tests/0.1/Testproject-tests-0.1.jar to repository remote at ...
Transferring 2K from remote
Uploaded 2K
Uploading: org/myorg/Testproject-tests/0.1/Testproject-tests-0.1-sources.jar to repository remote at ...
Transferring 1K from remote
Uploaded 1K
Uploading: org/myorg/Testproject-tests/0.1/Testproject-tests-0.1-tests.jar to repository remote at ...
Transferring 1K from remote
Uploaded 1K

while other times

Uploading: org/myorg/Testproject-tests/0.1/Testproject-tests-0.1-tests.jar to repository remote at ...
Transferring 1K from remote
Uploaded 1K
Uploading: org/myorg/Testproject/0.1/Testproject-0.1.jar to repository remote at ...
Transferring 2K from remote
Uploaded 2K
Uploading: org/myorg/Testproject/0.1/Testproject-0.1-tests.jar to repository remote at ...
Transferring 1K from remote
Uploaded 1K
Uploading: org/myorg/Testproject/0.1/Testproject-0.1-sources.jar to repository remote at ...
Transferring 1K from remote
Uploaded 1K

In practice the inclusion of jars seems not always related to the match of filter condition. In particular often only one pom/artifactId have the right jars, while the other have all the jars.

I'm attaching a test project.
PS: the need to upload tests jars with a different artifactId arised from the fact that the STS gradle integration seems to have problems supporting dependencies on multiple jars for the same project, i.e. given this deps

dependencies {
    compile 'org.myorg:Testproject:latest.release'
    testCompile 'org.myorg:Testproject:latest.release:tests'

it somewhat discards the first dep (maybe it's related to the fact that eclipse/sts uses the same classpath for tests and the main code, hence somewhat overwriting the two deps... I have to investigate further to understand if it's a bug of sts/gradle integration)