Unit test for verifying complexity of basic transformer

[ashawley]

A new unit test -- taken from code example in SI-4528 -- to verify changes in #58 for maintaining linear complexity of scala.xml.transform.BasicTransformer.transform.

Adding this test is motivated on refactoring of #58 to remove a compiler warning -- see #83.

Here is how I verified the unit test of this PR. I cherry-picked it on to the revision before the fix in #58 (b2eef1f), and then after (master). As you can see, it fails for before, but succeeds on the current master.

$ git checkout -b complexity-before b2eef1f^
Switched to a new branch 'complexity-before'
$ git cherry-pick SI-4528-transformer-complexity
[complexity-before bd4dd2b] Unit test for verifying complexity of basic transformer
 Date: Tue Oct 13 11:34:42 2015 -0400
 1 file changed, 22 insertions(+), 1 deletion(-)
$ sbt "testOnly scala.xml.Transformers"
[error] Test scala.xml.Transformers.preserveReferentialComplexityInLinearComplexity failed: expected:<1> but was:<32>
[error] Failed: Total 4, Failed 1, Errors 0, Passed 3
[error] Failed tests:
[error]     scala.xml.Transformers
[error] (test:testOnly) sbt.TestsFailedException: Tests unsuccessful
[error] Total time: 4 s, completed Nov 3, 2015 10:32:30 PM
$ git checkout -b complexity-after master
Switched to a new branch 'complexity-after'
$ git cherry-pick SI-4528-transformer-complexity
[complexity-after cb5dd60] Unit test for verifying complexity of basic transformer
 Date: Tue Oct 13 11:34:42 2015 -0400
 1 file changed, 22 insertions(+), 1 deletion(-)
$ sbt "testOnly scala.xml.Transformers"
[info] Passed: Total 4, Failed 0, Errors 0, Passed 4
[success] Total time: 4 s, completed Nov 3, 2015 9:39:50 PM
$ git checkout master
Switched to branch 'master'
$ git branch -D complexity-before
Deleted branch complexity-before (was bd4dd2b).
$ git branch -D complexity-after
Deleted branch complexity-after (was cb5dd60).

[biswanaths]

Pull the PR local, verified and pushed to the master.