Arrange unified /bin <-> $(bindir) in test-suite root

Arrange unified /bin <-> $(bindir) in test-suite root

- Rpm traditionally insists on putting itself into /bin and expects
  to find various things, especially /bin/sh there as well. This
  is normally the case, but on systems where the host /bin is symlink
  to eg /usr/bin, the test-suite /bin will be empty except for rpm
  itself causing a big number of bogus test-suite failures. Making
  test-suite root bin/ always a symlink to $(bindir) avoids this issue
  and works whether the host /bin is a real directory or not.