Add lowercase tests for >NUMBER

[Prism019]

After using this test suite to test the Forth-2012 System I was developing and it passing the Core tests, I wrote a program that relied on parsing numbers with >NUMBER that had lowercase digits in bases larger than 10, which failed on my System. Section 3.2.1.2 of the Forth-2012 Standard specifies that lowercase digits are treated as if they were uppercase digits.

This PR adds four tests to the >NUMBER section that are similar to the last four presently there, testing parsing of lowercase digits in bases larger than 10. I tested these in gForth 0.7.9 and my System.

I don't know if this is the preferred way to implement these tests. If having lowercase characters between the ticks is unwanted, my other idea was to make another word named GNLOWER' that is identical to GN', except for adding 0x20 to the char before storing it into GN-BUF.