hash: Close or flush sha256 output ports before calling their 'get' procedure.

hash: Close or flush sha256 output ports before calling their 'get' procedure.

Slightly different buffering of custom binary output ports in Guile 2.1.7
would lead tests missing 'close-port' or 'force-output' to fail.

* tests/hash.scm ("open-sha256-port, hello"): Add call to 'force-output'
before call to 'get'.
* tests/nar.scm ("write-file puts file in C locale collation order"):
Call 'close-port' before calling 'get-hash'.