Remove _test directory

[jtieri]

We have almost reached feature parity in ibctest with the existing integration style tests in the relayer. The only test that is missing over in ibctest is the test for multi-channel relaying on a single connection.

We can start to remove the tests and the associated GitHub actions for these tests in the relayer and once the last test is complete we can kill the entire directory which will remove the need to run mv _test test && go mod tidy && mv test _test when trying to adjust the go.mod file.