Tests for TestTextfilePageGenerator

Amend documentation for -file to mention titles may be
separated by new lines.

Also adds TestCaseBase.assertPagelistTitles to check the
loaded results from TextfilePageGenerator and other
pagegenerators.

Change-Id: I20bb62247e6b0b8d7a13a425cde210e60eb7d730
Merge "Tests for TestTextfilePageGenerator"