Add a separate test for BCE

This test is properly gated on tmux 2.4.

src/tests/Makefile.am

@@ -20,6 +20,7 @@ displaytests = \
 	emulation-attributes-256color8.test \
 	emulation-attributes-256color248.test \
 	emulation-attributes-truecolor.test \
+	emulation-attributes-bce.test \
 	emulation-back-tab.test \
 	emulation-cursor-motion.test \
 	emulation-multiline-scroll.test \

src/tests/emulation-attributes-bce.test

@@ -0,0 +1 @@
+emulation-attributes.test

src/tests/emulation-attributes.test

@@ -20,6 +20,12 @@ if [ "$(basename "$0")" = emulation-attributes-truecolor.test ] &&
     printf "tmux does not support true color\n" >&2
     exit 77
 fi
+# Need 2.4 for BCE support
+if [ "$(basename "$0")" = emulation-attributes-bce.test ] &&
+   ! tmux_check 2 4; then
+    printf "tmux does not support BCE\n" >&2
+    exit 77
+fi
 
 # Top-level wrapper.
 if [ $# -eq 0 ]; then
@@ -117,6 +123,14 @@ baseline()
             echo "Bold, italic and underline:"
             test_true_color 1 3 4
             ;;
+	# BCE in combination with various color modes.
+	bce)
+	    # True color.
+	    printf '\033[48;2;255;0;255m\033[H\033[JTrue color\n'
+	    printf '\033[48;5;32m\033[J256 color\n'
+	    printf '\033[42m\033[J16 color\n'
+	    printf '\033[0mdone\n'
+	    ;;
 	*)
 	    fail "unknown test name %s\n" "$1"
 	    ;;