By building the posix library as shared object with an ABI, we
effectively decouple posix-using programs from the library
implementation (which happens to depend on several os-level APIs such as
the VFS).
This test reproduces an issue of the VFS block file system when the
underlying block device has a coarser granularity than the block
requests issued by the VFS client. I.e., if the underlying block device
has a block size of 4K, writing a sequence of (non-4K-aligned) 512 blocks
that crosss a 4K boundary corrupts the data on the block device.
Issue #2262
