SDL: Mention the tests in INSTALL.md

From 7d2a1c5f8fb5d21a6c82b0e281cefa23272aa802 Mon Sep 17 00:00:00 2001
From: Sam Lantinga <[EMAIL REDACTED]>
Date: Sun, 12 Jan 2025 18:49:13 -0800
Subject: [PATCH] Mention the tests in INSTALL.md

---
 INSTALL.md | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/INSTALL.md b/INSTALL.md
index 52e044156c651..f04c4bed528bd 100644
--- a/INSTALL.md
+++ b/INSTALL.md
@@ -21,6 +21,8 @@ There are simple example programs in the examples directory, and you can view th
 
 https://examples.libsdl.org/SDL3
 
+More in-depth test programs are available in the tests directory and can be built by adding `-DSDL_TESTS=ON` to the CMake command line when building SDL.
+
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