Add test for Javascript literal with an unpaired surrogate character

Author: smowton

javascript/ql/test/library-tests/literals/test.expected

@@ -0,0 +1 @@
+| test.js:1:9:1:16 | "\\ud800" | \ufffd | "\\ud800" | \ufffd | test.js:1:9:1:16 | "\\ud800" |

javascript/ql/test/library-tests/literals/test.js

@@ -0,0 +1 @@
+let s = "\ud800";

javascript/ql/test/library-tests/literals/test.ql

@@ -0,0 +1,5 @@
+import javascript
+
+from StringLiteral sl
+where sl.getFile().getBaseName() = "test.js"
+select sl, sl.getValue(), sl.getRawValue(), sl.getStringValue(), sl.getUnderlyingValue()