SPARK-2955 [BUILD] Test code fails to compile with "mvn compile" without "install"

(This is the corrected follow-up to https://issues.apache.org/jira/browse/SPARK-2903)

Right now, `mvn compile test-compile` fails to compile Spark. (Don't worry; `mvn package` works, so this is not major.) The issue stems from test code in some modules depending on test code in other modules. That is perfectly fine and supported by Maven.

It takes extra work to get this to work with scalatest, and this has been attempted: https://github.com/apache/spark/blob/master/sql/catalyst/pom.xml#L86

This formulation is not quite enough, since the SQL Core module's tests fail to compile for lack of finding test classes in SQL Catalyst, and likewise for most Streaming integration modules depending on core Streaming test code. Example:

```
[error] /Users/srowen/Documents/spark/sql/core/src/test/scala/org/apache/spark/sql/QueryTest.scala:23: not found: type PlanTest
[error] class QueryTest extends PlanTest {
[error]                         ^
[error] /Users/srowen/Documents/spark/sql/core/src/test/scala/org/apache/spark/sql/CachedTableSuite.scala:28: package org.apache.spark.sql.test is not a value
[error]   test("SPARK-1669: cacheTable should be idempotent") {
[error]   ^
...
```

The issue I believe is that generation of a `test-jar` is bound here to the `compile` phase, but the test classes are not being compiled in this phase. It should bind to the `test-compile` phase.

It works when executing `mvn package` or `mvn install` since test-jar artifacts are actually generated available through normal Maven mechanisms as each module is built. They are then found normally, regardless of scalatest configuration.

It would be nice for a simple `mvn compile test-compile` to work since the test code is perfectly compilable given the Maven declarations.

On the plus side, this change is low-risk as it only affects tests.
@yhuai made the original scalatest change and has glanced at this and thinks it makes sense.