Add basic driver tests

Driver tests test the compilation of an entire file or collection
of files, from generating executables to linking libraries. These
are particularly helpful for testing compatibility on different
platforms.

Author: treemcgee42

cobalt-cli/src/tests/inputs/hello_world.co

@@ -1,4 +1,6 @@
+@C(extern) fn puts(str: *u8);
+
 fn main(): i32 = {
-    hello_world();
+    puts("Hello, world!"c);
     0
 };

cobalt-cli/src/tests/inputs/hello_world_linked_main.co

@@ -0,0 +1,4 @@
+fn main(): i32 = {
+    hello_world();
+    0
+};