chore: remove entire VM test suite and update test README for CLI-only focus

Delete all VM-related test files including benchmark, call, error, foreign_class, and other API tests, along with their C source, headers, and Wren scripts. Rewrite test/README.md to describe only CLI unit tests and module resolution tests, removing references to VM, compiler, core library, and language semantics tests.

test/module/logical_dir/wren_modules/bar/within_bar.wren

@@ -0,0 +1,5 @@
+// nontest
+var Bar = "from bar"
+System.print("ran bar module")
+
+import "foo/within_foo"

test/module/logical_dir/wren_modules/foo/within_foo.wren

@@ -0,0 +1,5 @@
+// nontest
+var Foo = "from foo"
+System.print("ran foo module")
+
+import "bar/within_bar"