Add lexer test for escape sequence

Tests/StencilTests/LexerSpec.swift

@@ -99,5 +99,18 @@ class LexerTests: XCTestCase {
         }
       }
     }
+
+    $0.it("can tokenize escape sequences") {
+      let templateString = "class Some {{ '{' }}{% if true %}{{ stuff }}{% endif %}"
+      let lexer = Lexer(templateString: templateString)
+      let tokens = lexer.tokenize()
+
+      try expect(tokens.count) == 5
+      try expect(tokens[0]) == Token.text(value: "class Some ", at: SourceMap(location: lexer.rangeLocation(templateString.range(of: "class Some ")!)))
+      try expect(tokens[1]) == Token.variable(value: "'{'", at: SourceMap(location: lexer.rangeLocation(templateString.range(of: "'{'")!)))
+      try expect(tokens[2]) == Token.block(value: "if true", at: SourceMap(location: lexer.rangeLocation(templateString.range(of: "if true")!)))
+      try expect(tokens[3]) == Token.variable(value: "stuff", at: SourceMap(location: lexer.rangeLocation(templateString.range(of: "stuff")!)))
+      try expect(tokens[4]) == Token.block(value: "endif", at: SourceMap(location: lexer.rangeLocation(templateString.range(of: "endif")!)))
+    }
   }
 }