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/use_nearest_modules_dir/a/b/use_nearest_modules_dir.wren

@@ -0,0 +1,3 @@
+// Stops as soon as it finds a wren_modules directory, regardless of whether or
+// not it contains the desired module.
+import "foo" // expect runtime error: Could not load module 'foo'.

test/module/use_nearest_modules_dir/a/wren_modules/empty.txt

@@ -0,0 +1 @@
+This file exists just to make Git create the containing directory.

test/module/use_nearest_modules_dir/wren_modules/foo/foo.wren

@@ -0,0 +1,2 @@
+// nontest
+System.print("ran foo module")