meson: Do not use path separator in test names
meson: Do not use path separator in test names

Avoiding warnings like:

    WARNING: Target "elements/audioamplify" has a path separator in its name.

StreamUtils/gst-plugins-ugly,StreamUtils/gst-plugins-ugly,GrokImageCompression/gst-plugins-ugly,GStreamer/gst-plugins-ugly,GrokImageCompression/gst-plugins-ugly,GStreamer/gst-plugins-ugly,StreamUtils/gst-plugins-ugly,GStreamer/gst-plugins-ugly,GrokImageCompression/gst-plugins-ugly