Add tests for updating plugins

tests/test_plugin_update.sh

@@ -0,0 +1,35 @@
+#!/usr/bin/env bash
+
+CURRENT_DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
+
+source $CURRENT_DIR/helpers.sh
+
+manually_install_the_plugin() {
+	mkdir -p ~/.tmux/plugins/
+	cd ~/.tmux/plugins/
+	git clone --quiet https://github.com/tmux-plugins/tmux-example-plugin
+}
+
+test_plugin_installation() {
+	set_tmux_conf_helper <<- HERE
+	set -g @tpm_plugins "tmux-plugins/tmux-example-plugin"
+	run-shell "~/tpm/tpm"
+	HERE
+
+	manually_install_the_plugin
+
+	# opens tmux and test it with `expect`
+	$CURRENT_DIR/expect_successful_update_of_all_plugins ||
+		fail_helper "Tmux 'update all plugins' fails"
+
+	$CURRENT_DIR/expect_successful_update_of_a_single_plugin ||
+		fail_helper "Tmux 'update single plugin' fails"
+
+	teardown_helper
+}
+
+main() {
+	test_plugin_installation
+	exit_value_helper
+}
+main