build: fix shunloadtest breakage

build: fix shunloadtest breakage

Commit 93ec384 was tested on mingw, but broke the build on Linux:

  CCLD     shunloadtest
shunloadtest.o: In function `main':
/home/eblake/libvirt/tests/shunloadtest.c:106: undefined reference to `virFilePrintf'
...
ssh.o: In function `main':
/home/eblake/libvirt/tests/ssh.c:43: undefined reference to `virFilePrintf'
/home/eblake/libvirt/tests/ssh.c:49: undefined reference to `virFilePrintf'

* tests/testutils.h (fprintf): Provide escape hatch.
* tests/shunloadtest.c: Use it.
* tests/ssh.c: Likewise.

Signed-off-by: Eric Blake <eblake@redhat.com>