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
```
