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/contains.wren

@@ -0,0 +1,8 @@
+io.write("".contains(""))              // expect: true
+io.write("anything".contains(""))      // expect: true
+io.write("something".contains("meth")) // expect: true
+io.write("something".contains("some")) // expect: true
+io.write("something".contains("ing"))  // expect: true
+io.write("something".contains("math")) // expect: false
+
+// TODO(bob): Passing non-string as argument.