Extracted boilerplate from Groovy perf tests

This commit reduces the length of GroovySerialTest by about 160 lines
by moving helper methods into GroovyTestSupport and
LongIterator(.groovy).  Some of the helper methods in
GroovyTestSupport are now unused and will be removed in a later
commit.
Read/write Groovy test graph from a file

This commit uses Blueprints's GraphMLWriter and -Reader classes to
dump/load a file containing relations.  The schema is not saved
anywhere.  The schema is effectively hardcoded as the defaults in
c.t.t.testutil.gen.Schema.  The Schema settings (and more) used to
generate the v10k.graphml.gz file in this commit are described in a
comment near the top of GroovySerialTest.

This commit also refactors GroovySerialTest and GroovyTestSupport to
reduce a bit more boilerplate code in GroovySerialTest's domain logic.