Tests failing in Linux: Package xkbcommon-x11 not found

### What are you trying to do?

Checkout Gonum, run all the tests

### What did you do?

```
$ go test ./...
```

### What did you expect to happen?

OK

### What actually happened?

```
$ go test ./...
go: downloading gioui.org v0.0.0-20210308172011-57750fc8a0a6
go: downloading golang.org/x/sys v0.0.0-20210304124612-50617c2ba197
ok gonum.org/v1/plot 0.434s
ok gonum.org/v1/plot/cmpimg 0.948s
ok gonum.org/v1/plot/font 0.051s
? gonum.org/v1/plot/font/liberation [no test files]
ok gonum.org/v1/plot/gob 0.086s
ok gonum.org/v1/plot/palette 0.168s
? gonum.org/v1/plot/palette/brewer [no test files]
ok gonum.org/v1/plot/palette/moreland 2.450s
# pkg-config --cflags -- egl wayland-egl wayland-client wayland-cursor x11 xkbcommon xkbcommon-x11 x11-xcb xcursor xfixes
Package xkbcommon-x11 was not found in the pkg-config search path.
Perhaps you should add the directory containing `xkbcommon-x11.pc'
to the PKG_CONFIG_PATH environment variable
No package 'xkbcommon-x11' found
pkg-config: exit status 1
ok gonum.org/v1/plot/plotter 10.454s
...
...
FAIL
```

### What version of Go and Gonum/plot are you using?

```
$ go version
go version go1.16 linux/amd64
$ git rev-parse HEAD
caab7788e04dfd7a74cec4ee80f55b7d1f9a2bfa
```
This is Gonum v0.9.0

### Does this issue reproduce with the current master?

Yes

Inizia eseguendo `go test ./...` su Linux e analizza il fallimento di pkg-config per `xkbcommon-x11`; il payload non indica alcun file sorgente né alcun test oltre all’intera suite. Determina quale dipendenza dei test richiede questo pacchetto e documenta o correggi la configurazione dei test su Linux in modo che l’intera suite passi.

Scritto dal modello di indicizzazione a partire dal testo della issue.