Add tests for % and $ chars in atoms

Signed-off-by: Rudi Grinberg <rudi.grinberg@gmail.com>

test/unit-tests/sexp.mlt

@@ -107,3 +107,33 @@ parse {|"\%{x}"|}
 - : parse_result =
 Different {jbuild = Ok ["\\%{x}"]; dune = Error "unknown escape sequence"}
 |}]
+
+parse {|"$foo"|}
+[%%expect{|
+- : parse_result = Same (Ok ["$foo"])
+|}]
+
+parse {|"%foo"|}
+[%%expect{|
+- : parse_result = Same (Ok ["%foo"])
+|}]
+
+parse {|"bar%foo"|}
+[%%expect{|
+- : parse_result = Same (Ok ["bar%foo"])
+|}]
+
+parse {|"bar$foo"|}
+[%%expect{|
+- : parse_result = Same (Ok ["bar$foo"])
+|}]
+
+parse {|"%bar$foo%"|}
+[%%expect{|
+- : parse_result = Same (Ok ["%bar$foo%"])
+|}]
+
+parse {|"$bar%foo%"|}
+[%%expect{|
+- : parse_result = Same (Ok ["$bar%foo%"])
+|}]