We’re trying to eliminate the legacy `tests/` directory. This commit
moves the code from `tests/iochannel-test.c` into
`glib/tests/io-channel.c` and ports it to the latest GLib test coding
standards:
* Change `g_assert()` to `g_assert_*()`
* Print verbose messages with `g_test_message()`
* Rename some variables to conform to modern conventions
* Use `GTest`
Signed-off-by: Philip Withnall <pwithnall@endlessos.org>
Helps: #1434