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/assignment/global.wren

@@ -0,0 +1,8 @@
+var a = "before"
+io.write(a) // expect: before
+
+a = "after"
+io.write(a) // expect: after
+
+io.write(a = "arg") // expect: arg
+io.write(a) // expect: arg

test/assignment/grouping.wren

@@ -0,0 +1,2 @@
+var a = "a"
+(a) = "value" // expect error

test/assignment/infix_operator.wren

@@ -0,0 +1,3 @@
+var a = "a"
+var b = "b"
+a + b = "value" // expect error

test/assignment/is.wren

@@ -0,0 +1,3 @@
+var a = "a"
+var b = "b"
+b is a = "value" // expect error

test/assignment/local.wren

@@ -0,0 +1,10 @@
+fn {
+  var a = "before"
+  io.write(a) // expect: before
+
+  a = "after"
+  io.write(a) // expect: after
+
+  io.write(a = "arg") // expect: arg
+  io.write(a) // expect: arg
+}.call

test/assignment/prefix_operator.wren

@@ -0,0 +1,2 @@
+var a = "a"
+!a = "value" // expect error

test/assignment/syntax.wren

@@ -0,0 +1,11 @@
+// Chained assignment.
+var a = "a"
+var b = "b"
+a = b = "chain"
+io.write(a) // expect: chain
+io.write(a) // expect: chain
+
+// Assignment on RHS of variable.
+var c = a = "var"
+io.write(a) // expect: var
+io.write(c) // expect: var

test/assignment/undefined.wren

@@ -0,0 +1 @@
+unknown = "what" // expect error