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/equality.wren

@@ -0,0 +1,19 @@
+io.write("" == "")          // expect: true
+io.write("abcd" == "abcd")  // expect: true
+io.write("abcd" == "d")     // expect: false
+io.write("e" == "abcd")     // expect: false
+io.write("" == "abcd")      // expect: false
+
+// Not equal to other types.
+io.write("1" == 1)        // expect: false
+io.write("true" == true)  // expect: false
+
+io.write("" != "")          // expect: false
+io.write("abcd" != "abcd")  // expect: false
+io.write("abcd" != "d")     // expect: true
+io.write("e" != "abcd")     // expect: true
+io.write("" != "abcd")      // expect: true
+
+// Not equal to other types.
+io.write("1" != 1)        // expect: true
+io.write("true" != true)  // expect: true