[Libguestfs] [PATCH v2] build: Drop serial_tests.

[Richard W.M. Jones]

I pushed the (hopefully) completely non-controversial bits
upstream:

https://github.com/libguestfs/libguestfs/commit/8a72616bf7bc686ad4d033482541fcd73c148b53
https://github.com/libguestfs/libguestfs/commit/b20d36aa1bcabfe1e5eefcf47b727280a6474be8

This patch is what remains.

Rich.