Add support for Sdump.

This commit adds a function named Sdump which works exactly like Dump and
Fdump except it returns the formatted output as a string.  This serves the
same purpose as the the Sprint* family of functions.
Add tests for new Sdump function.

This commit adds tests for the new Sdump function both at the package
level and as a part of a ConfigState.
Update test coverage report for new Sdump func.