Add tests for issue 157

- `''` escapes `$` in both single- and double-quoted strings.
- `$$` cannot start an antiquote.
- In a double-quoted string, `\$$` cannot start an antiquote.
- In a single-quoted string, `\$$` *can* start an antiquote.

Author: nosewings

tests/nix-font-lock-tests.el

@@ -81,6 +81,30 @@ if all of its characters have syntax and face. See
      ("with" t nix-keyword-face)
      ("foo" t nix-attribute-face))))
 
+(ert-deftest nix-issue-157 ()
+  (check-properties
+   '("\"''$${x}")
+   '(("\"''$${x}" t font-lock-string-face)))
+  (check-properties
+   '("''''$${x}''")
+   '(("''''$" t font-lock-string-face)
+     ("${" t nix-antiquote-face)
+     ("}" t nix-antiquote-face)))
+  (check-properties
+   '("\"$${x}\"")
+   '(("\"$${x}\"" t font-lock-string-face)))
+  (check-properties
+   '("''$${x}''")
+   '(("''$${x}''" t font-lock-string-face)))
+  (check-properties
+   '("\"\\$${x}\"")
+   '(("\"\\$" t font-lock-string-face)
+     ("${" t nix-antiquote-face)
+     ("}" t nix-antiquote-face)))
+  (check-properties
+   '("''\\$${x}''")
+   '(("''\\$${x}''" t font-lock-string-face))))
+
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