Add UI tests for string escape warnings.

src/test/ui/str/str-escape.rs

@@ -0,0 +1,11 @@
+// check-pass
+fn main() {
+    let s = "\
+
+             ";
+    //~^^^ WARNING multiple lines skipped by escaped newline
+    let s = "foo\
+             bar
+             ";
+    //~^^^ WARNING non-ASCII whitespace symbol '\u{a0}' is not skipped
+}

src/test/ui/str/str-escape.stderr

@@ -0,0 +1,21 @@
+warning: multiple lines skipped by escaped newline
+  --> $DIR/str-escape.rs:3:14
+   |
+LL |       let s = "\
+   |  ______________^
+LL | |
+LL | |              ";
+   | |_____________^ skipping everything up to and including this point
+
+warning: non-ASCII whitespace symbol '\u{a0}' is not skipped
+  --> $DIR/str-escape.rs:7:17
+   |
+LL |       let s = "foo\
+   |  _________________^
+LL | |              bar
+   | |   ^ non-ASCII whitespace symbol '\u{a0}' is not skipped
+   | |___|
+   | 
+
+warning: 2 warnings emitted
+