support/testing: fix s6-portable-utils test

s6-test was removed in s6-portable-utils 2.3.0.0, and replaced with
eltest (from execline). This change broke the existing runtime tests
with commit 42fad03182 ("packages: bump skalibs-related packages").

Instead of s6-test simply use the shell built-in "test", which provides
sufficient functionality.

Fixes:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/7338388010

Signed-off-by: J. Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

support/testing/tests/package/test_s6_portable_utils.py

@@ -28,5 +28,5 @@ class TestS6PortableUtils(infra.basetest.BRTest):
 
         _, exit_code = self.emulator.run("s6-mkfifo testpipe")
         self.assertEqual(exit_code, 0)
-        _, exit_code = self.emulator.run("s6-test -p testpipe")
+        _, exit_code = self.emulator.run("test -p testpipe")
         self.assertEqual(exit_code, 0)