chore: reorganize test files into domain-specific subdirectories

- Moved unit tests for user authentication into `tests/unit/auth/`
- Relocated integration tests for payment processing to `tests/integration/payments/`
- Grouped API endpoint tests under `tests/api/` with separate files for each resource
- Updated test runner configuration to discover tests in new directory structure

test/string/subscript.wren

@@ -0,0 +1,24 @@
+// Returns characters (as strings).
+io.write("abcd"[0]) // expect: a
+io.write("abcd"[1]) // expect: b
+io.write("abcd"[2]) // expect: c
+io.write("abcd"[3]) // expect: d
+
+// Allows indexing backwards from the end.
+io.write("abcd"[-4]) // expect: a
+io.write("abcd"[-3]) // expect: b
+io.write("abcd"[-2]) // expect: c
+io.write("abcd"[-1]) // expect: d
+
+// Handle out of bounds.
+// TODO(bob): Should halt the fiber or raise an error somehow.
+io.write("abcd"[4])  // expect: null
+io.write("abcd"[-5]) // expect: null
+
+// Handle wrong argument type.
+// TODO(bob): Should halt the fiber or raise an error somehow.
+io.write("abcd"[true]) // expect: null
+
+// Handle non-integer index.
+// TODO(bob): Should halt the fiber or raise an error somehow.
+io.write("abcd"[1.5]) // expect: null