Fix port availability test (#333)

* Let OS allocate port number

Port availability might be tricky if some port is already in use on the
'all' interface '0.0.0.0'.

The proposed solution is to let the OS allocate the port for us using 0
as port specifier :

- first create a server instance to allocate a port number
- save port number
- close the server
- return the port number to the caller

This should be safe granted the OS doesn't reuse the port numbers on
consecutive port allocations.

see :
- about Node.js Net module : https://nodejs.org/docs/latest-v12.x/api/net.html#net_server_listen_port_host_backlog_callback
- about safety around reusing port number : https://unix.stackexchange.com/a/132524

Signed-off-by: Alexis Deruelle <alexis.deruelle@gmail.com>

src/main/routes/port-forward.ts

@@ -36,7 +36,7 @@ class PortForward {
   }
 
   public async start() {
-    this.localPort = await getFreePort(8000, 9999)
+    this.localPort = await getFreePort()
     const kubectlBin = await bundledKubectl.kubectlPath()
     const args = [
       "--kubeconfig", this.kubeConfig,