test-container: Use nftw instead of rm -rf

If the binary to run is 'env', test-containers skips it and adds
any required environment variable on the process envs variables.
This simplifies the required code to spawn new process (no need
to build an env-like program).

However, this is an issue for recursive_remove if there is any
LD_PRELOAD, since test-container will not prepend the loader command
along with required paths.  If the required preloaded library can
not be loaded by the system glibc, the 'post-clean rsync' will
eventually fail.

One example is if system glibc does not support DT_RELR and the
built glibc does, the nss/tst-nss-gai-hv2-canonname test fails
with:

../scripts/evaluate-test.sh nss/tst-nss-gai-hv2-canonname $? false false
86_64-linux-gnu/nss/tst-nss-gai-hv2-canonname.test-result
rm: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_ABI_DT_RELR' not
found (required by x86_64-linux-gnu/malloc/libc_malloc_debug.so)

Instead trying to figure out the required loader arguments on how
to spawn the 'rm -rf', replace the command with a nftw call.

Checked on x86_64-linux-gnu.
Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
Reviewed-by: Stefan Liebler <stli@linux.ibm.com>