test(test_description_t): temp nagfor workaround

This commit uses the NAG compiler's predefined NAGFOR macro to
remove two lines in test_description_test.F90 to work around a
bug in the nagfor compiler. The compiler bug has been isolated and
reported and the associated macro conditionals can be removed once
a patched version of the compiler is available. In the interim,
simply substituting character variables for string_t arguments in
the test's contains_text check works around the issue.

test/test_description_test.F90

@@ -128,10 +128,14 @@ function check_substring_search() result(tests_pass)
       ,test_description_t("nothing to see here"      , unused) &
     ]
     tests_pass = [ &
-             test_descriptions(1)%contains_text(string_t("example")       ) &
-      ,      test_descriptions(2)%contains_text(         "example"        ) &
-      ,.not. test_descriptions(3)%contains_text(string_t("missing string")) &
-      ,.not. test_descriptions(4)%contains_text(         "missing string" ) &
+#ifndef NAGFOR
+             test_descriptions(1)%contains_text(string_t("example")       ), &
+#endif
+             test_descriptions(2)%contains_text(         "example"        ), &
+#ifndef NAGFOR
+       .not. test_descriptions(3)%contains_text(string_t("missing string")), &
+#endif
+       .not. test_descriptions(4)%contains_text(         "missing string" ) &
     ]
   end function