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/e2e-test-subrs

@@ -54,3 +54,24 @@ chr()
 {
     printf '%b' "\\0$(printf %03o "$1")"
 }
+
+# If the locale is not set to a UTF-8 locale, set it to en_US.UTF-8
+# or C.UTF-8.
+set_locale()
+{
+    # Test for a usable locale.
+    map=$(locale charmap 2>/dev/null)
+    if [ "$map" = "utf-8" ] || [ "$map" = "UTF-8" ]; then
+	return 0
+    fi
+    # Attempt to find/set a usable locale.
+    for i in en_US.UTF-8 en_US.utf8 C.UTF-8; do
+	map="$(env LANG=$i locale charmap 2>/dev/null)"
+	if [ "$map" = "utf-8" ] || [ "$map" = "UTF-8" ]; then
+	    export LANG=$i LC_ALL=''
+	    return 0
+	fi
+    done
+    # Fail.
+    return 1
+}