Avoid fall-through in test-container if execlp fails.

One of the implicit-fallthrough warnings from compiling glibc with
-Wextra appears to indicate an actual bug: the test-container code
could fall through inappropriately if execlp returns (which only
occurs on error).  This patch adds appropriate error handling in this
case to avoid that fall-through.

Tested for x86_64.

	* support/test-container.c (recursive_remove): Use FAIL_EXIT1 if
	execlp returns.