increase stack size when running JUnit tests
increase stack size when running JUnit tests

scala.collection.immutable.PagedSeqTest.test_SI6615 was failing
intermittently during PR validation

I verified that this change fixes the problem by:
- changing the PageSize constant in PagedSeq (as shown by
  Jason Zaugg) until the test failed every time locally
- making this build change and seeing the test pass again

5M is just an arbitrary number, considerably over the default (which
varies according to platform & CPU). 5M is a lot of stack, but not so
vastly much that it appreciably eats into the heap

lrytz/scala,jvican/scala,scala/scala,shimib/scala,slothspot/scala,martijnhoekstra/scala,martijnhoekstra/scala,slothspot/scala,felixmulder/scala,shimib/scala,jvican/scala,felixmulder/scala,scala/scala,shimib/scala,lrytz/scala,slothspot/scala,scala/scala,jvican/scala,felixmulder/scala,felixmulder/scala,scala/scala,felixmulder/scala,martijnhoekstra/scala,shimib/scala,lrytz/scala,jvican/scala,shimib/scala,jvican/scala,slothspot/scala,martijnhoekstra/scala,shimib/scala,scala/scala,lrytz/scala,martijnhoekstra/scala,slothspot/scala,lrytz/scala,lrytz/scala,felixmulder/scala,slothspot/scala,felixmulder/scala,jvican/scala,scala/scala,jvican/scala,slothspot/scala,martijnhoekstra/scala