feat(test): Add unit tests for all themes

Use Tmux itself by spinning up a temporary test server, load the theme,
and use "show-options" to pull out and validate relevant options that
are set by the theme.

test/version_test.go

@@ -0,0 +1,25 @@
+package test
+
+import (
+	"bytes"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestTmuxVersion(t *testing.T) {
+	tmuxSetup()
+	defer tmuxTearDown()
+
+	if tmuxVersion != "" {
+		out, err := tm.Exec("-V")
+		require.NoError(t, err)
+
+		assert.Equal(
+			t,
+			"tmux "+tmuxVersion,
+			string(bytes.TrimSpace(out)),
+		)
+	}
+}