lib: metadata: transform fast path precond. checks to BT_ASSERT_PRE()

lib: metadata: transform fast path precond. checks to BT_ASSERT_PRE()

Use BT_ASSERT_PRE() to assert that trace, stream, stream class, event
class, and field type preconditions are satisfied instead of
unconditional run-time checks for typical fast path API functions
(getters).

Update `tests/lib/test_ctf_writer.c` accordingly to remove unit tests
which expect a specific return value when preconditions are not
satisfied.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>