patch 9.0.1656: syntax test fails when detected shell type differs

Problem:    Syntax test fails when detected shell type differs.
Solution:   Avoid using "/bin/sh", it depends on the system.  Add a check that
            the shell type detection is correct.

runtime/syntax/testdir/input/sh_01.sh

@@ -1,4 +1,4 @@
-#! /bin/sh
+#! /bin/dash
 export `echo 'A=B'`
 printenv A
 echo a `#foo` b