Add basic tests for the appmenu

My main motivation for writing this was as a regression test for my
recent patch "appmenu: require exact match with picked"
https://lists.sr.ht/~mil/sxmo-devel/patches/32362

Signed-off-by: Anjandev Momi <anjan@momi.ca>

scripts/core/sxmo_appmenu.sh

@@ -144,4 +144,7 @@ mainloop() {
 	fi
 }
 
-mainloop "$@"
+# Allow loading from shellspec
+if [ -z "$SHELLSPEC_PATH" ]; then
+	mainloop "$@"
+fi

spec/scripts/core/sxmo_appmenu_spec.sh

@@ -0,0 +1,60 @@
+# shellcheck disable=SC2155
+# SPDX-License-Identifier: AGPL-3.0-only
+# Copyright 2022 Sxmo Contributors
+
+# Don't check for unused variables
+# shellcheck disable=SC2034
+
+Describe 'sxmo_appmenu.sh'
+	Include scripts/core/sxmo_appmenu.sh
+
+	# if the script successfully exits, it confuses shellspec
+	quit() {
+		true
+	}
+
+	getprogchoices() {
+		true
+	}
+
+	It 'can mock functions'
+		CHOICES="foo ^ 0 ^ echo bar"
+		Mock sxmo_dmenu.sh
+			echo 'foo '
+		End
+
+		When call mainloop
+
+		The output should equal 'bar'
+		The stderr should match pattern '*' # ignore stderr
+		The status should be success
+	End
+
+	It 'ignores invalid selections'
+		CHOICES="foo ^ 0 ^ echo bar"
+		Mock sxmo_dmenu.sh
+			echo 'baz '
+		End
+
+		When call mainloop
+
+		The output should equal ''
+		The stderr should match pattern '*' # ignore stderr
+		The status should be success
+	End
+
+	It 'handles substring menu items'
+		CHOICES="foobar ^ 0 ^ echo foobar
+bar ^ 0 ^ echo bar2"
+
+		Mock sxmo_dmenu.sh
+			echo 'bar '
+		End
+
+		When call mainloop
+
+		The output should equal 'bar2'
+		The stderr should match pattern '*' # ignore stderr
+		The status should be success
+	End
+End