Tests should run with C locale

Dear indent authors / Hello Andrej.

While preparing a new Debian upload for indent I noticed
that it only built ok inside a sid chroot, but not under
Debian stable (!)

A closer look tells me the reason:

$ cat regression/output/bug206785.err-diffs
1,2c1,2
<	indent: input/bug206785.c:22: Warning:Unterminated string constant
<	indent: input/bug206785.c:22: Error:Unexpected end of file
---
>	indent: input/bug206785.c:22: Atención:Constante de cadena sin terminar
>	indent: input/bug206785.c:22: Error:Fin de fichero inesperado

So, I have to add LC_ALL=C somewhere in debian/rules, but maybe such
setting should be in regression/TEST instead.

Thanks.