tests/int: relax testPids fork error match string

The test checked for the exact BusyBox ash diagnostic "sh: can't fork".
With BusyBox 1.38, ash reports the failure as:

  /bin/sh: line 0: can't fork: Resource temporarily unavailable

Match the stable "can't fork" part of the error message instead.

Signed-off-by: Ricardo Branco <rbranco@suse.de>
(cherry picked from commit de39d5e79b)
Signed-off-by: Ricardo Branco <rbranco@suse.de>

libcontainer/integration/exec_test.go

@@ -558,7 +558,7 @@ func testPids(t *testing.T, systemd bool) {
 	// this to fail reliably.
 	config.Cgroups.Resources.PidsLimit = mkPtr[int64](64)
 	out, _, err := runContainer(t, config, "/bin/sh", "-c", truePipeline(64))
-	if err != nil && !strings.Contains(out.String(), "sh: can't fork") {
+	if err != nil && !strings.Contains(out.String(), "can't fork") {
 		t.Fatal(err)
 	}