Add a custom test runner

src/main.rs

@@ -1,5 +1,8 @@
 #![no_std]
 #![no_main]
+#![feature(custom_test_frameworks)]
+#![test_runner(crate::test_runner)]
+#![reexport_test_harness_main = "test_main"]
 
 use core::panic::PanicInfo;
 
@@ -9,9 +12,20 @@ mod vga_buffer;
 pub extern "C" fn _start() -> ! {
     println!("Hello World{}", "!");
 
+    #[cfg(test)]
+    test_main();
+
     loop {}
 }
 
+#[cfg(test)]
+fn test_runner(tests: &[&dyn Fn()]) {
+    println!("Running {} tests", tests.len());
+    for test in tests {
+        test();
+    }
+}
+
 /// This function is called on panic.
 #[panic_handler]
 fn panic(info: &PanicInfo) -> ! {