Hello Frederic
If you are using SPAN to capture your packets, in order to capture control packets such as those used for STP (and for CDP, VTP, and others) you must use the encapsulation replicate keywords at the end of the destination port command. Take a look at this post for more info.
But you must keep in mind that for emulation programs such as GNS3 and Virl, from a short search that I have done, I see that most users are unable to capture those multicast packets. It could be simply a limitation of the emulators. You’ll need real equipment to capture them.
You can see an example of such captured packets at this Wireshark packet capture:
https://wiki.wireshark.org/SampleCaptures?action=AttachFile&do=get&target=STP+UplinkFast.pcapng
The specific packets you want to look at are those that state STP-UplinkFast in the Destination MAC column of the list of packets.
I hope this has been helpful!
Laz