testing/capnproto: add missing depend on libucontext-dev
The CMake file tries to link to ucontext:
/usr/lib/cmake/CapnProto/CapnProtoTargets.cmake: INTERFACE_LINK_LIBRARIES "ucontext;pthread"
But the linker can't find the .so to link to:
ld: cannot find -lucontext
Edited by Luca Weiss