Only consider letters when checking if a name is upper-case

Previously, for a string to be considered upper-case,
EmbeddedServerPortFileWriter required every character in the
string to be upper-case. This meant that strings containing numbers were
considered lower-case even if every letter in the string was upper-case.
OS X’s case-preserving, case-insensitive file system masked this problem
as the tests were still able to find the created file, even though the
case of its name was not as expected.

This commit updates EmbeddedServerPortFileWriter to only require
characters that are letters (as defined by Character.isLetter()) to be
upper-case. It also updates the tests to verify that the case of the
created file’s name is correct in such a way that it will fail, even
on OS X, when it is not.

Fixes gh-1676