check.gstreamer.elements_leaks.test_get_live_objects fails sometimes on the CI

`check.gstreamer.elements_leaks.test_get_live_objects`

Command

GST_CHECKS='test_get_live_objects' CK_DEFAULT_TIMEOUT='20' GST_STATE_IGNORE_ELEMENTS=''  GST_PLUGIN_LOADING_WHITELIST='gstreamer' GST_PLUGIN_SCANNER_1_0='/builds/gdesmott/gstreamer/gst-build/build/subprojects/gstreamer/libs/gst/helpers/gst-plugin-scanner' CK_TIMEOUT_MULTIPLIER='2.0' GST_REGISTRY='/builds/gdesmott/gstreamer/gst-build/build/registry.dat' GST_PLUGIN_SYSTEM_PATH_1_0='' /builds/gdesmott/gstreamer/gst-build/build/subprojects/gstreamer/tests/check/elements_leaks

elements_leaks output

Running suite(s): leakstracer
0%: Checks: 1, Failures: 1, Errors: 0
../subprojects/gstreamer/tests/check/elements/leaks.c:184:F:live-objects:test_get_live_objects:0: 'size' (3) is not equal to 'NUM_BUFFERS' (2)
Check suite leakstracer ran in 0.029s (tests failed: 1)

You can mark the issues as 'known' by adding the following lines to the list of known issues

    "FIXME 'check.gstreamer.elements_leaks.test_get_live_objects' issues [REPORT A BUG in https://gitlab.freedesktop.org/gstreamer/ or use a proper bug description]": {
        "tests": [
            "check.gstreamer.elements_leaks.test_get_live_objects"
        ],
        "issues": [
            {
                'returncode': 1,
                'sometimes': True,
            },
        ],
    },

**Duration**: 0.581780195236206
==> check.gstreamer.elements_leaks.test_get_live_objects: Failed 'Application returned 1'

Artifacts from the CI run: artifacts.zip (since they will get deleted in a couple of days).

First noticed at https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/487#note_522189

# `check.gstreamer.elements_leaks.test_get_live_objects`
## Command
``` bash
GST_CHECKS='test_get_live_objects' CK_DEFAULT_TIMEOUT='20' GST_STATE_IGNORE_ELEMENTS=''  GST_PLUGIN_LOADING_WHITELIST='gstreamer' GST_PLUGIN_SCANNER_1_0='/builds/gdesmott/gstreamer/gst-build/build/subprojects/gstreamer/libs/gst/helpers/gst-plugin-scanner' CK_TIMEOUT_MULTIPLIER='2.0' GST_REGISTRY='/builds/gdesmott/gstreamer/gst-build/build/registry.dat' GST_PLUGIN_SYSTEM_PATH_1_0='' /builds/gdesmott/gstreamer/gst-build/build/subprojects/gstreamer/tests/check/elements_leaks
```
## elements_leaks output
``` 
Running suite(s): leakstracer
0%: Checks: 1, Failures: 1, Errors: 0
../subprojects/gstreamer/tests/check/elements/leaks.c:184:F:live-objects:test_get_live_objects:0: 'size' (3) is not equal to 'NUM_BUFFERS' (2)
Check suite leakstracer ran in 0.029s (tests failed: 1)
```
**You can mark the issues as 'known' by adding the  following lines to the list of known issues**
``` python
    "FIXME 'check.gstreamer.elements_leaks.test_get_live_objects' issues [REPORT A BUG in https://gitlab.freedesktop.org/gstreamer/ or use a proper bug description]": {
        "tests": [
            "check.gstreamer.elements_leaks.test_get_live_objects"
        ],
        "issues": [
            {
                'returncode': 1,
                'sometimes': True,
            },
        ],
    },
```
```
**Duration**: 0.581780195236206
==> check.gstreamer.elements_leaks.test_get_live_objects: Failed 'Application returned 1'
```

Artifacts from the CI run: [artifacts.zip](/uploads/9c5e5c86fc1322f33ed9254e8530f243/artifacts.zip) (since they will get deleted in a couple of days).