artifact from //:defs.bzl doesn't handle test jars

When specifying dependencies for JVM build targets using `artifact` from `defs.bzl`, it does not appear to have a mechanism to specify a test jar. Ideally there would be a parallel to what `maven.artifact` provides so that maven style coordinates can be used throughout. As an example a test jar can be referenced using maven.artifact as follows:
`maven.artifact("org.apache.flink", "flink-streaming-java_2.11", FLINK_VERSION, classifier="tests"),`.

I have worked around the issue with the following solution however it would be preferable to consistently use maven coordinates throughout the workspace:
```
java_junit5_test(
name = "test",
srcs = glob(["*.java"]),
test_package = "com.example.foobar",
deps = [
artifact("org.apache.flink:flink-streaming-java_2.11"),
"@maven//:org_apache_flink_flink_streaming_java_2_11_tests",
... etc
],
)
```

The following 3 examples are what I could see as solutions:
```
# use tests type (maven style coordinate specification of classifier)
artifact("org.apache.flink:flink-streaming-java_2.11:tests"),

# use explicit flag that matches maven.artifact
artifact("org.apache.flink", "flink-streaming-java_2.11", FLINK_VERSION, classifier="tests"),

# use include_tests which pulls creates dependencies on both jar and the test jar.
artifact("org.apache.flink:flink-streaming-java_2.11", include_tests=True),
```

Example maven artifacts:
https://repo1.maven.org/maven2/org/apache/flink/flink-streaming-java_2.11/1.11.2/

Start from the artifact entry point in defs.bzl and compare its dependency handling with maven.artifact, especially the classifier example in the issue. Done means artifact can consistently reference the Maven test jar for the provided Flink coordinates without the separate @maven dependency workaround.

Written by the indexing model from the issue text.