emp: use TCPServer instead of WEBrick for test

[stefanb]

Related:

• go 1.21.7 #161976
• go 1.22.0 #157782

both have a problem with webrick test:

  An exception occurred within a child process:
    LoadError: cannot load such file -- webrick

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[chenrui333]

  ==> Testing emp
  Error: emp: failed
  An exception occurred within a child process:
    LoadError: cannot load such file -- webrick

[SMillerDev]

Related to remind101/empire#1167

[stefanb]

If this works then a similar fix should probably be applied also to few other formulae.

[stefanb]

cc @p-linnane & @iMichka