refac(string_test): reorg/narrow GCC workaround

This commit
1. Reorganizes the block of test code that contains a gfortran
   workaround in a way that priorities readability over avoiding
   code duplication and
2. Uses the GCC_VERSION preprocessor macro to apply the workaround
   only for GCC versions below 15.1.

Author: rouson

test/modules/string_test_m.F90

@@ -571,23 +571,27 @@ function brackets_strings() result(test_diagnosis)
 
     associate(scalar => string_t("do re mi"))
 
-#ifndef __GFORTRAN__
+#if (! defined(__GFORTRAN__)) || GCC_VERSION > 150000
       associate(array  => string_t(["do", "re", "mi"]))
+        test_diagnosis = test_diagnosis_t( &
+          test_passed = scalar%bracket()        == string_t("[do re mi]")                                  &
+               .and. all(array%bracket()        == [string_t("[do]"), string_t("[re]"), string_t("[mi]")]) &
+               .and. all(array%bracket('"')     == [string_t('"do"'), string_t('"re"'), string_t('"mi"')]) &
+               .and. all(array%bracket("{","}") == [string_t('{do}'), string_t('{re}'), string_t('{mi}')]) &
+          ,diagnostics_string = "" &
+        )
+      end associate
 #else
       block
         type(string_t), allocatable :: array(:)
         array = string_t(["do", "re", "mi"])
-#endif
-      test_diagnosis = test_diagnosis_t( &
-        test_passed = scalar%bracket()        == string_t("[do re mi]")                                  &
-             .and. all(array%bracket()        == [string_t("[do]"), string_t("[re]"), string_t("[mi]")]) &
-             .and. all(array%bracket('"')     == [string_t('"do"'), string_t('"re"'), string_t('"mi"')]) &
-             .and. all(array%bracket("{","}") == [string_t('{do}'), string_t('{re}'), string_t('{mi}')]) &
-        ,diagnostics_string = "" &
-      )
-#ifndef __GFORTRAN__
-      end associate
-#else
+        test_diagnosis = test_diagnosis_t( &
+          test_passed = scalar%bracket()        == string_t("[do re mi]")                                  &
+               .and. all(array%bracket()        == [string_t("[do]"), string_t("[re]"), string_t("[mi]")]) &
+               .and. all(array%bracket('"')     == [string_t('"do"'), string_t('"re"'), string_t('"mi"')]) &
+               .and. all(array%bracket("{","}") == [string_t('{do}'), string_t('{re}'), string_t('{mi}')]) &
+          ,diagnostics_string = "" &
+        )
       end block
 #endif
     end associate