libstdc++: testsuite: Enhance codecvt_unicode with tests for length()

We can test codecvt::length() with the same data that we test
codecvt::in(). For each call of in() we add another call to length().
Some additional small cosmentic changes are applied.

libstdc++-v3/ChangeLog:

	* testsuite/22_locale/codecvt/codecvt_unicode.h: Test length()