fstests: refactor test boilerplate code

fstests: refactor test boilerplate code

Create two new helper functions to deal with boilerplate test code:

A helper function to set the seq and seqnum variables.  We will expand
on this in the next patch so that fstests can autogenerate group files
from now on.

A helper function to register cleanup code that will run if the test
exits or trips over a standard range of signals.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Chandan Babu R <chandanrlinux@gmail.com>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>