Improve randomness of uploaded file names and files created by tempnam()

Closes GH-14364

Fix skipif condition on new test

I'm an idiot, messed up the condition when tweaking the test manually before committing.

Fix off-by-one bug when truncating tempnam prefix

The tempnam documentation currently states that "Only the first 63
characters of the prefix are used, the rest are ignored". However whe

Fix off-by-one bug when truncating tempnam prefix

The tempnam documentation currently states that "Only the first 63
characters of the prefix are used, the rest are ignored". However when
the prefix is 64 characters-long, the current implementation fails to
strip the last character, diverging from the documented behavior. This
patch fixes the implementation so it matches the documented behavior for
that specific case where the prefix is 64 characters long.

Closes GH-11870

Signed-off-by: George Peter Banyard <girgias@php.net>

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