fmt: improve test coverage of %x and %X format variations for strings

fmt: improve test coverage of %x and %X format variations for strings

The tests in the basic string section are now covering more code paths
for encoding a string into the hexadecimal representation of its bytes.

Changed the basic string and basic bytes tests so that they mirror each other.

Change-Id: Ib5dc7b33876769965f9aba2ac270040abc4b2451
Reviewed-on: https://go-review.googlesource.com/2611
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Rob Pike <r@golang.org>