lntest: avoid global ServeMux

[torkelrogstad]

Using the default, global ServeMux prevents the same process from
calling lntest.NewNetworkHarness multiple times, because we get a
panic when registering HTTP routes.

Instead, we use the ServeMux belonging to the fee service struct.

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  I could add a new test that calls lntest.NewNetworkHarness twice, but that sounds like a slow/expensive test for little benefit...
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[philipglazman]

Ran into this issue as well. Confirming your PR fixes the root issue.

[torkelrogstad]

Possible to get this in @Crypt-iQ @Roasbeef, after I update the release notes?

[guggero]

Thanks for the fix, LGTM 🎉

[yyforyongyu]

LGTM⚡️