curlhttpsrc: test_multiple_http_requests unit test is flaky

The test_multiple_http_requests fails regularly on the CI, so it has been blacklisted.

Unexpected critical/warning: 
Trying to dispose element fakesink1, but it is in READY instead of the NULL state.
You need to explicitly set elements to the NULL state before
dropping the final reference, to allow them to clean up.
This problem may also be caused by a refcounting bug in the
application or some element.


Stack trace:
gst_debug_get_stack_trace (gstinfo.c:2886)
gst_check_log_critical_func (gstcheck.c:281)
g_logv (/usr/lib64/libglib-2.0.so.0.5800.3:0x7fb45543a4fb)
g_log (/usr/lib64/libglib-2.0.so.0.5800.3:0x7fb45543a6ef)
g_object_unref (/usr/lib64/libgobject-2.0.so.0.5800.3:0x7fb4553a48c4)
gst_bin_remove_func (gstbin.c:1809)
gst_bin_remove (gstbin.c:1871)
gst_bin_dispose (gstbin.c:527)
g_object_unref (/usr/lib64/libgobject-2.0.so.0.5800.3:0x7fb4553a48c4)
gst_bin_remove_func (gstbin.c:1809)
gst_bin_remove (gstbin.c:1871)
gst_bin_dispose (gstbin.c:527)
g_object_unref (/usr/lib64/libgobject-2.0.so.0.5800.3:0x7fb4553a48c4)
test_multiple_http_requests (curlhttpsrc.c:656)
srunner_run_tagged (check_run.c:465)
gst_check_run_suite (gstcheck.c:1067)
main (curlhttpsrc.c:693)
__libc_start_main (/usr/lib64/libc-2.28.so:0x7fb45501e40f)
_start (/builds/gstreamer/gst-plugins-bad/gst-build/build/subprojects/gst-plugins-bad/tests/check/elements_curlhttpsrc:0x40369a)

0%: Checks: 1, Failures: 1, Errors: 0
../subprojects/gstreamer/libs/gst/check/gstcheck.c:286:F:general:test_multiple_http_requests:0: Unexpected critical/warning: 
Trying to dispose element fakesink1, but it is in READY instead of the NULL state.
You need to explicitly set elements to the NULL state before
dropping the final reference, to allow them to clean up.
This problem may also be caused by a refcounting bug in the
application or some element.

(also see !181 (merged) for similar issues)

CC @alexashley