nixos/testing: disallow special chars in machine names in network expressions

These names are referenced by Perl variables inside the testing
frameworks which don't allow chars like `-` as character inside. An exemplary
expression may look like this:

```
{
  x11-vm = {
    services.xserver.enable = true;
  };
}
```

This expression evaluates, e.g. when running `nixos-build-vms`, but when
trying to run `./result/bin/nixos-run-vms`, an error like this occurs:

```
starting VDE switch for network 1
running the VM test script
error: Can't modify subtraction (-) in scalar assignment at (eval 17) line 1, at EOF
Bareword "test" not allowed while "strict subs" in use at (eval 17) line 1.
Can't modify subtraction (-) in scalar assignment at (eval 17) line 1, at EOF
Bareword "test" not allowed while "strict subs" in use at (eval 17) line 1.
vde_switch: EOF on stdin, cleaning up and exiting
cleaning up
```

This can be very confusing for beginners, this change breaks evaluation
if such names are used for machines.

nixos/lib/testing.nix

@@ -149,9 +149,23 @@ in rec {
       test = passMeta (runTests driver);
       report = passMeta (releaseTools.gcovReport { coverageRuns = [ test ]; });
 
-    in (if makeCoverageReport then report else test) // {
+      nodeNames = builtins.attrNames nodes;
-      inherit nodes driver test;
+      invalidNodeNames = lib.filter
-    };
+        (node: builtins.match "^[A-z_][A-z0-9_]+$" node == null) nodeNames;
+
+    in
+      if lib.length invalidNodeNames > 0 then
+        throw ''
+          Cannot create machines out of (${lib.concatStringsSep ", " invalidNodeNames})!
+          All machines are references as perl variables in the testing framework which will break the
+          script when special characters are allowed.
+
+          Please stick to alphanumeric chars and underscores as separation.
+        ''
+      else
+        (if makeCoverageReport then report else test) // {
+          inherit nodes driver test;
+        };
 
   runInMachine =
     { drv