Add missing "x11/vaapi" to the s-r-s-i report

In the !123 (merged) I forgot to also print VA-API to the JSON report.

I added a test for the list of graphics that we expect to have. When I finish to work on parsing the JSON output, hopefully it will be easier to spot problems like this one.

This is a sample output:

      "graphics-details" : {
        "x11/vulkan" : {
          "renderer" : "AMD Radeon RX 5700 XT",
          "version" : "1.2.135 (device 1002:731f) (driver 2.0.140)"
        },
        "x11/vdpau" : {
          "renderer" : "G3DVL VDPAU Driver Shared Library version 1.0\n",
          "version" : null
        },
        "x11/vaapi" : {
          "renderer" : "Mesa Gallium driver 20.0.5 for AMD Radeon RX 5700 XT (NAVI10, DRM 3.36.0, 5.6.7-arch1-1, LLVM 10.0.0)\n",
          "version" : null
        },
        "glx/gl" : {
          "renderer" : "AMD Radeon RX 5700 XT (NAVI10, DRM 3.36.0, 5.6.7-arch1-1, LLVM 10.0.0)",
          "version" : "4.6 (Compatibility Profile) Mesa 20.0.5",
          "library-vendor" : "glvnd"
        },
[...]

/cc @smcv @jpwhiting