libct/int: make TestFdLeaks more robust

The purpose of this test is to check that there are no extra file
descriptors left open after repeated calls to runContainer. In fact,
the first call to runContainer leaves a few file descriptors opened,
and this is by design.

Previously, this test relied on two things:
1. some other tests were run before it (and thus all such opened-once
   file descriptors are already opened);
2.  explicitly excluding fd opened to /sys/fs/cgroup.

Now, if we run this test separately, it will fail (because of 1 above).
The same may happen if the tests are run in a random order.

To fix this, add a container run before collection the initial fd list,
so those fds that are opened once are included and won't be reported.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/integration/exec_test.go

@@ -1679,6 +1679,16 @@ func testFdLeaks(t *testing.T, systemd bool) {
 		return
 	}
 
+	config := newTemplateConfig(t, &tParam{systemd: systemd})
+	// Run a container once to exclude file descriptors that are only
+	// opened once during the process lifetime by the library and are
+	// never closed. Those are not considered leaks.
+	//
+	// Examples of this open-once file descriptors are:
+	//  - /sys/fs/cgroup dirfd opened by prepareOpenat2 in libct/cgroups;
+	//  - dbus connection opened by getConnection in libct/cgroups/systemd.
+	_ = runContainerOk(t, config, "true")
+
 	pfd, err := os.Open("/proc/self/fd")
 	ok(t, err)
 	defer pfd.Close()
@@ -1687,7 +1697,6 @@ func testFdLeaks(t *testing.T, systemd bool) {
 	_, err = pfd.Seek(0, 0)
 	ok(t, err)
 
-	config := newTemplateConfig(t, &tParam{systemd: systemd})
 	_ = runContainerOk(t, config, "true")
 
 	fds1, err := pfd.Readdirnames(0)
@@ -1699,7 +1708,6 @@ func testFdLeaks(t *testing.T, systemd bool) {
 	// Show the extra opened files.
 
 	excludedPaths := []string{
-		"/sys/fs/cgroup",      // opened once, see prepareOpenat2
 		"anon_inode:bpf-prog", // FIXME: see https://github.com/opencontainers/runc/issues/2366#issuecomment-776411392
 	}