Undeclared dependency on rtpmanager in tests

Hi,

meson test fails on elements_rtpsrc and elements_rtpsink when gst-plugins-good is installed to a non-standard directory (but available on GST_SYSTEM_PLUGIN_PATH, etc), because tests/check/meson.build does not make rtpmanager available.

The meson.build checks for gstreamer and gst-plugins-base with pkg-config and adds them on GST_PLUGIN_PATH_1_0. Something similar is needed for gst-plugins-good, but it currently lacks a pkg-config file.