Add test matrix for ghdlsynth

[eine]

In order to do so, yosys is required. Moreover, the test suite of yosys requires iverilog.

https://github.com/1138-4EB/ghdl/tree/gcc-in-docker

[eine]

Docker images for icestorm, nextpnr, yosys and symbiyosys were added in https://github.com/ghdl/docker/commit/4b57cc40ea800d880ef4ebf6431ee596caac94a6.

• Image ghdl/synth:latest supports ghdl --synth only.
• Image ghdl/synth:beta includes yosys and ghdlsynth as a module too.
• Image ghdl/synth:formal includes symbiyosys on top of ghdl/synth:beta.

Currently, ghdl --synth is tested in the main repo. ghdlsynth is tested in tgingold/ghdlsynth-beta and here. Precisely, the travis.sh script from there is executed here in order to generate latest and beta images; and test the latter only.