extmod/vfs_posix: Fix accidentally passing tests.

These tests test an unrealistic situation and only pass by accident due
to a bug. The upcoming fix for the bug would make them fail.

The unrealistic situation is that VfsPosix methods are called with
relative paths while the current working directory is somewhere outside
of the root of the VFS. In the intended use of VFS objects via
os.mount() (as opposed to calling methods directly as the tests do),
this never happens, as mp_vfs_lookup_path() directs incoming calls to
the VFS that contains the CWD.

Make the testing situation realistic by changing the working directory
to the root of the VFS before calling methods on it, as the subsequent
relative path accesses expect.

Thanks to the preceding commit, the tests still pass, but still for the
wrong reason. The following commit "Fix relative paths on non-root VFS"
will make them pass for the correct reason.

Signed-off-by: Christian Walther <cwalther@gmx.ch>

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