Pattern for valid environment variable names as defined in
The Open Group Base Specifications Issue 7, 2018 edition
IEEE Std 1003.1-2017 (Revision of IEEE Std 1003.1-2008)
https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html
Updated tests to match UPPERCASE ONLY var names.