HDFFV-10128 fix string compare and add tests

tools/test/h5diff/testfiles/h5diff_61.txt

@@ -0,0 +1,30 @@
+dataset: </string2> and </string2>
+size:           [20]           [20]
+position        string2         string2         difference          
+------------------------------------------------------------
+[ 8 ]            9            e
+[ 8 ]                         f
+[ 8 ]                         9
+[ 9 ]            0            e
+[ 9 ]                         f
+[ 9 ]                         0
+[ 10 ]            1            e
+[ 10 ]                         f
+[ 10 ]                         1
+[ 11 ]            2            e
+[ 11 ]                         f
+[ 11 ]                         2
+[ 12 ]            e            3
+[ 12 ]            f             
+[ 12 ]            3             
+[ 13 ]            e            4
+[ 13 ]            f             
+[ 13 ]            4             
+[ 14 ]            e            5
+[ 14 ]            f             
+[ 14 ]            5             
+[ 15 ]            e            6
+[ 15 ]            f             
+[ 15 ]            6             
+24 differences found
+EXIT CODE: 1