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.