Reorganize tests and benchmark scripts.

Mainly to get rid of one top level directory. But this will
also be useful when there are tests of the embedding API.

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