test: always use bash

If sh is not bash, some builtin command behave slightly differently.
E.g. if sh is provided by busybox, its builtin test command does not check if
the path is a mount point or not, and 'test -w' only checks the access mode of
the inode. So, even if a readonly filesystem is mounted on a directory,
the test command may succeed.

To avoid such confusion, let's unconditionally use bash.

test/test-execute/exec-standardinput-data.service

@@ -3,7 +3,7 @@
 Description=Test for StandardInputText= and StandardInputData=
 
 [Service]
-ExecStart=sh -x -c 'd=$$(mktemp -d -p /tmp); echo -e "this is a test\nand this is more\nsomething encoded!\nsomething   in multiple lines\nand some more\nand a more bas64 data\nsomething with strange\nembedded\tcharacters\nand something with a exec-stdin-data.service specifier" >$d/text ; cmp $d/text ; rm -rf $d'
+ExecStart=bash -x -c 'd=$$(mktemp -d -p /tmp); echo -e "this is a test\nand this is more\nsomething encoded!\nsomething   in multiple lines\nand some more\nand a more bas64 data\nsomething with strange\nembedded\tcharacters\nand something with a exec-stdin-data.service specifier" >$d/text ; cmp $d/text ; rm -rf $d'
 Type=oneshot
 StandardInput=data
 StandardInputText=this is a test