Minor rewording of some sections of the ERT manual.

Minor rewording of some sections of the ERT manual.

* doc/misc/ert.texi (Introduction, How to Run Tests)
(Running Tests Interactively, Expected Failures)
(Tests and Their Environment, Useful Techniques)
(Interactive Debugging, Fixtures and Test Suites):
Minor rephrasings.
(Running Tests Interactively, The @code{should} Macro): Add xrefs.
(Running Tests in Batch Mode): Simplify loading instructions.
(Test Selectors): Clarify some selectors.
(Expected Failures, Useful Techniques):
Make examples fit in 80 columns.
Remove unused "LocalWords".