Ensure a UTF-8 locale is available for tests

Fixes Debian sid schroot build.

Also add build deps required for full testing:
tmux,  less (for window-resize.test)

src/tests/local.test

@@ -1,4 +1,11 @@
 #!/bin/sh
+
+. "$(dirname "$0")/e2e-test-subrs"
+if ! set_locale; then
+    echo "$0: no usable locale" >&2
+    exit 99
+fi
+
 set -eu
 out=$(
     TERM=xterm \