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_04.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/dash
 # sh4
 Variable=${VariableB:-{VariableC}}
 Variable=${VariableB:-${VariableC:-{Var3:=eng}}}