tests: provide a second cpu core

Provides a small (~7.5%) reduction in the test runtime measured for the external
test:

Before:
```
Benchmark 1: nix build .#hydraJobs.x86_64-linux.external-unstable --rebuild
  Time (mean ± σ):     151.737 s ±  1.074 s    [User: 0.310 s, System: 0.289 s]
  Range (min … max):   150.321 s … 153.512 s    10 runs
```

After:
```
Benchmark 1: nix build .#hydraJobs.x86_64-linux.external-unstable --rebuild
  Time (mean ± σ):     140.647 s ±  1.092 s    [User: 0.331 s, System: 0.296 s]
  Range (min … max):   138.536 s … 142.298 s    10 runs
```

tests/lib/config.nix

@@ -1,4 +1,12 @@
+{
+  lib,
+  ...
+}:
+
 {
   # Testing eval failures that result from stateVersion assertion is out of scope
   mailserver.stateVersion = 999;
+
+  # Enable second CPU core
+  virtualisation.cores = lib.mkDefault 2;
 }