refactor(run): remove I/O view test navigation keymaps

Problem: <c-n>/<c-p> in the I/O view buffers required the cursor
to leave the source file to work, re-ran the solution on each
press, and gave no indication of which test was active. The
workflow is better served by :CP run <n> for a specific test or
:CP panel for full inspection.

Solution: remove navigate_test, next_test_key/prev_test_key config
options, and the associated current_test_index state field.

lua/cp/state.lua

@@ -9,7 +9,6 @@
 ---@class cp.IoViewState
 ---@field output_buf integer
 ---@field input_buf integer
----@field current_test_index integer?
 ---@field source_buf integer?
 
 ---@class cp.State