test: Move a test from search to search-output, and add a similar json test

test: Move a test from search to search-output, and add a similar json test

The "Search for non-existent message prints nothing" test fits better
with the existing tests in search-output, so move it there. Also add a
similar test for the --format=json case.

These tests also use the new test_expect_equal_file function, (to ensure
that the presence of a trailing newline is correctly tested).