Add basic testsuite using ui_test

tests/ui/function_calls_ok.nil

@@ -0,0 +1,19 @@
+//@check-pass
+
+function main() = (
+    noArgs();
+    singleArg("hi!");
+    manyArgs(1,2,3,4,5,6);
+
+    let a: () = returnNothing();
+    let b: () = returnExplicitUnit();
+    let c: String = returnString();
+);
+
+function noArgs() =;
+function singleArg(a: String) =;
+function manyArgs(a: Int, b: Int, c: Int, d: Int, e: Int, f: Int) =;
+
+function returnNothing() =;
+function returnExplicitUnit(): () =;
+function returnString(): String = "uwu";

tests/ui/hello_world.nil

@@ -0,0 +1,2 @@
+//@check-pass
+function main() = print("Hello, world!\n");

tests/ui/type_assignments.nil

@@ -0,0 +1,21 @@
+//@check-pass
+
+type CustomType = {};
+
+function main() = ();
+
+function assigns(
+    a: Int,
+    b: I32,
+    c: String,
+    d: Bool,
+    e: CustomType,
+    f: (Int, I32),
+) = (
+    let a1: Int = a;
+    let b1: I32 = b;
+    let c1: String = c;
+    let d1: Bool = d;
+    let e1: CustomType = e;
+    let f1: (Int, I32) = f;
+);