Add test SQLite KV extension

This extension mimics Limbo's kv_store and is used in tests to verify
that Limbo and SQLite handle extensions in a compatible way.

testing/sqlite_test_ext/build.rs

@@ -0,0 +1,11 @@
+fn main() {
+    let mut build = cc::Build::new();
+    build.file("src/kvstore.c");
+    build.include("include");
+    if !cfg!(windows) {
+        build.flag("-Wno-unused-parameter");
+    }
+    build.compile("kvstore");
+
+    println!("cargo:rerun-if-changed=src/kvstore.c");
+}