patch 9.0.1507: assert message is confusing with boolean result

Problem:    Assert message is confusing with boolean result.  assert_inrange()
            replaces message instead of adding it.
Solution:   Don't put quotes around expected boolean value.  Append message
            for assert_inrange(). (closes #12342, closes #12341)

src/vim.h

@@ -2254,6 +2254,7 @@ typedef enum {
     ASSERT_NOTEQUAL,
     ASSERT_MATCH,
     ASSERT_NOTMATCH,
+    ASSERT_FAILS,
     ASSERT_OTHER
 } assert_type_T;