spark-core depends on spark-tags (compile scope) which depends on scalatest
(compile scope), so spark-core leaks test-deps into downstream libraries'
"compile"-scope classpath.
The cause is that spark-core has logical "test->test" and
"compile->compile" dependencies on spark-tags, but spark-tags publishes
both its test-oriented and non-test-oriented bits in its default
("compile") artifact.
spark-tags' test-bits should be in a "-tests"-JAR that spark-core can
"test"-scope depend on (in addition to "compile"-scope depending on
spark-tags as it does today).
SPARK-17807 <https://issues.apache.org/jira/browse/SPARK-17807> was "Not a
Problem"d but I don't think that's the right outcome; spark-core should not
be leaking test-deps into downstream libraries' classpaths when depended on
in "compile" scope.
