We fix all linter issues except for the 'lostcontext' and 'unparam' ones
as those are too numerous and would increase the diff even more.
Therefore we silence them in the itest directory for now.
Because the linter is still not build tag aware, we also have to silence
the unused and deadcode sub linters to not get false positives.
In this commit, we modify our build tag set up to allow the main test
files to be buildable w/o the current rpctest tag. We do this so that
those of us that use extensions which will compile live files like
vim-go can once again fix compile errors as we go in our editors.
In order to do this, we now make an external `testsCases` variable, and
have two variants: one that's empty (no build tag), and one that's fully
populated with all our tests (build tag active). As a result, the main
file will now always build regardless of if the build tag is active or
not, but we'll only actually execute tests if the `testCases` variable
has been populated.
As sample run w/ the tag off:
```
=== RUN TestLightningNetworkDaemon
--- PASS: TestLightningNetworkDaemon (0.00s)
PASS
ok github.com/lightningnetwork/lnd/lntest/itest 0.051s
```
