Increase test timeout to six hours

The gokey tests takes 11408.183 s (= 190 min) on minkus.debian.org (mips
porter box):

```
ok      github.com/cloudflare/gokey     11408.183s
?       github.com/cloudflare/gokey/cmd/gokey   [no test files]
=== RUN   TestKnownKey
--- PASS: TestKnownKey (1473.20s)
PASS
ok      github.com/cloudflare/gokey/rsa 1473.222s
```

Therefore increase test timeout to six hours (in case of half as fast
mips machines).

Closes: #919759
Signed-off-by: Benjamin Drung <benjamin.drung@cloud.ionos.com>