lib/repo: Don't syncfs or fsync() dirs if fsync opt is disabled

There are use cases for not syncing at all; think build cache repos, etc. Let's
be consistent here and make sure if fsync is disabled we do no sync at all.

I chose this opportunity to add tests using the shiny new strace fault
injection.  I can forsee using this for a lot more things, so I made
the support for detecting things generic.

Related: https://github.com/ostreedev/ostree/issues/1184

Closes: #1186
Approved by: jlebon

ci/build.sh

@@ -10,7 +10,7 @@ pkg_upgrade
 pkg_install_builddeps ostree
 # Until this propagates farther
 pkg_install 'pkgconfig(libcurl)' 'pkgconfig(openssl)'
-pkg_install sudo which attr fuse \
+pkg_install sudo which attr fuse strace \
     libubsan libasan libtsan PyYAML redhat-rpm-config \
     elfutils
 if test -n "${CI_PKGS:-}"; then