refactor: move benchmark directory under test and update all references

The benchmark directory is relocated from the project root into the test directory, and all scripts, documentation links, and gitignore paths are updated accordingly. The test runner is also refactored to explicitly walk subdirectories (core, io, language, limit) instead of using a single test root, and a new test/README.md is added to document the test suite structure. Additionally, the constructor test for the Foo class is updated to add a zero-arity constructor and adjust expected output strings.

test/language/string/escapes.wren

@@ -0,0 +1,7 @@
+// Escape characters.
+IO.print("\"")     // expect: "
+IO.print("\\")     // expect: \
+IO.print("(\n)")   // expect: (
+                   // expect: )
+
+// TODO: Non-printing escapes like \t.

test/language/string/incomplete_unicode_escape.wren

@@ -0,0 +1,2 @@
+// expect error line 2
+"\u00"

test/language/string/incomplete_unicode_escape_at_eof.wren

@@ -0,0 +1,2 @@
+// expect error line 2
+"\u004

test/language/string/invalid_escape.wren

@@ -0,0 +1,2 @@
+// expect error line 2
+"not a \m real escape"

test/language/string/invalid_unicode_escape.wren

@@ -0,0 +1,2 @@
+// expect error line 2
+"\u00no"

test/language/string/literals.wren

@@ -0,0 +1,5 @@
+IO.print("".count)   // expect: 0
+IO.print("a string") // expect: a string
+
+// Non-ASCII.
+IO.print("A~¶Þॐஃ") // expect: A~¶Þॐஃ

test/language/string/unicode_escapes.wren

@@ -0,0 +1,17 @@
+// One byte UTF-8 Sequences.
+IO.print("\u0041")     // expect: A
+IO.print("\u007e")     // expect: ~
+
+// Two byte sequences.
+IO.print("\u00b6")     // expect: ¶
+IO.print("\u00de")     // expect: Þ
+
+// Three byte sequences.
+IO.print("\u0950")     // expect: ॐ
+IO.print("\u0b83")     // expect: ஃ
+
+// Capitalized hex.
+IO.print("\u00B6")     // expect: ¶
+IO.print("\u00DE")     // expect: Þ
+
+// TODO: Syntax for Unicode escapes > 0xffff?

test/language/string/unterminated.wren

@@ -0,0 +1,2 @@
+// expect error line 2
+"this string has no close quote