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-dynamicuser-fixeduser.service

@@ -3,8 +3,8 @@
 Description=Test DynamicUser with User=
 
 [Service]
-ExecStart=sh -x -c 'for g in $$(id -G); do test "$$g" = "1" && exit 0; done; exit 1'
-ExecStart=sh -x -c 'test "$$(id -g)" = "1" && test "$$(id -u)" = "1"'
+ExecStart=bash -x -c 'for g in $$(id -G); do test "$$g" = "1" && exit 0; done; exit 1'
+ExecStart=bash -x -c 'test "$$(id -g)" = "1" && test "$$(id -u)" = "1"'
 Type=oneshot
 User=1
 DynamicUser=yes