force LC_ALL=C.UTF-8 in basic-command-list test, so that diffoscope can always output something

I had an occasion when the test failed in a place because it thought the terminal
didn't support UTF-8, so forcing that.

debian/tests/basic-command-line

@@ -2,6 +2,9 @@
 
 # not set -e: we are testing exit codes
 
+# to have diffoscope able to output stuff in utf-8
+export LC_ALL=C.UTF-8
+
 if ! [ -d "$ADTTMP" ]; then
 	ADTTMP=`mktemp -d`
 	TEMP=true