## What changes were proposed in this pull request?
`ReplSuite.newProductSeqEncoder with REPL defined class` was flaky and throws OOM exception frequently. By analyzing the heap dump, we found the reason is that, in each test case of `ReplSuite`, we create a REPL instance, which creates a classloader and loads a lot of classes related to `SparkContext`. More details please see https://github.com/apache/spark/pull/17833#issuecomment-298711435.
In this PR, we create a new test suite, `SingletonReplSuite`, which shares one REPL instances among all the test cases. Then we move most of the tests from `ReplSuite` to `SingletonReplSuite`, to avoid creating a lot of REPL instances and reduce memory footprint.
## How was this patch tested?
test only change
Author: Wenchen Fan <wenchen@databricks.com>
Closes#17844 from cloud-fan/flaky-test.
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