tests: Modernize valgrind infrastructure

tests: Modernize valgrind infrastructure

The recent memleak fixes motivated me to look at the bitrotted code to
run invocations of `ostree` in the test suite underneath valgrind.

There are a few things here.  First, update suppressions file from
libhif, since I recently worked on it.

When running *uninstalled* as we now support, we need
`libtool --mode=execute` in the mix so it expands out to
the uninstalled binary and we don't valgrind the intermediate shell.

However, it's harder than that because we chdir into a tmpdir,
which defeats the libtool logic.  AFAICS, the only fix for this
is to determine the realbin path before we chdir, and then unfortunately
we need to change every use of `ostree` to `${OSTREE}` =(

Then this immediately breaks for me on RHEL7 because my ancient
copy of `valgrind-3.10.0-16.el7.x86_64` is unaware of syscall 306, i.e.
`syncfs`.

But let's do this first before I dive into that.

Closes: #292
Approved by: krnowak