Add proper tests for string_type concatenation

Author: Carl Burkert

src/tests/string/test_string_operator.f90

@@ -99,11 +99,13 @@ subroutine test_ne
     end subroutine test_ne
 
     subroutine test_concat
-        type(string_type) :: string
+        type(string_type) :: a, b
 
-        string = "Hello, "
-        string = string // "World!"
-        call check(len(string) == 13)
+        a = "a"
+        b = "b"
+        call check("a" //  b  == "ab")
+        call check( a  // "b" == "ab")
+        call check( a  //  b  == "ab")
     end subroutine test_concat
 
 end module test_string_operator