switchroot: Fix test-switchroot now autotools can build static

switchroot: Fix test-switchroot now autotools can build static

This test previously depended on manually building ostree-prepare-root.
Since 42dab85 we've been able to build static binaries with the usual
autotools build-system.  This change reflects the fact that
`ostree-prepare-root` is built into $srcdir rather than `src/switchroot`
where I was building manually.

This test now passes with `./configure --with-static-compiler=gcc` (glibc)
but still fails with `./configure --with-static-compiler=musl-gcc` (musl).

Closes: #485
Approved by: cgwalters