Fix: bt_ctfser_write_float64(): use `double` in union, not `float`

Fix: bt_ctfser_write_float64(): use `double` in union, not `float`

This bug made both CTF writer and `sink.ctf.fs` write wrong 64-bit
floating point fields.

This patch also adds two `sink.ctf.fs` tests to catch this. Both tests
run a CTF generator which uses CTF writer, then run

    babeltrace2 /path/to/generated/trace -o ctf -w /path/to/converted/trace

and then pass the converted trace into a `sink.text.details` sink to
verify that the content is expected.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: If69ee9a0e1d4038dd28a72860ecc11f18bb6b50e
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1645
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>