Add tests for updating plugins

tests/expect_successful_update_of_a_single_plugin

@@ -0,0 +1,61 @@
+#!/usr/bin/env expect
+
+# disables script output
+log_user 0
+
+spawn tmux
+
+# Waiting for tmux to attach. If this is not done, next command, `send` will
+# not work properly.
+sleep 1
+
+# this is tmux prefix + U
+send "U"
+
+set timeout 5
+
+expect {
+  "Installed plugins" {
+
+    expect {
+      "\"tmux-example-plugin\"" {
+
+        expect {
+          "\"all\" - updates all plugins" {
+
+            expect { "ENTER - cancels" }
+
+          }
+        }
+
+      }
+    }
+
+  }
+
+  timeout {
+    puts "Plugin update prompt timeout";
+    exit 1
+  }
+}
+
+send "tmux-example-plugin\r"
+
+expect {
+  "Updating \"tmux-example-plugin\"" {
+
+    expect {
+      "\"tmux-example-plugin\" update success" {
+
+        expect { "Done, press ENTER to continue." { exit 0 } }
+
+      }
+    }
+
+  }
+
+  timeout {
+    puts "Update all plugins timeout";
+    exit 1
+  }
+}