tests/qtest/migration: Split TLS tests from migration-test.c

tests/qtest/migration: Split TLS tests from migration-test.c

The migration-test.c file has become unwieldy large. It's quite
confusing to navigate with all the test definitions mixed with hook
definitions. The TLS tests make this worse with ifdef'ery.

Since we're planning on having a smaller set of tests to run as smoke
testing on all architectures, I'm taking the time to split some tests
into their own file.

Move the TLS tests into a file of their own.

Acked-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>