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

runtime/syntax/testdir/input/sh_04.sh

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

runtime/syntax/testdir/input/sh_07.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/dash
 # Test file to test 'for do done' loops.
 # You can start this script like: $0 {-ne -gt -le ...} (all numeric operators 
 # are allowed!

runtime/syntax/testdir/input/sh_09.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/dash
 # Test file for vim the check () subshells
 ( cd ; $pwd ) | wc -c
 ( cd $1 ; $pwd ) | wc -c