tests: Reset macOS dyld environment

tests: Reset macOS dyld environment

This is needed to ensure the environment variables that we need
for the test program itself, specifically to load mock libraries,
do not interfere with any command that gets invoked by it, either
directly or indirectly. We already perform the same cleanup step
for LD_* variables.

This makes the test failures

  error : virCommandWait:2752 : internal error: Child process
    (/usr/libexec/qemu/vhost-user/test-vhost-user-gpu --print-capabilities)
    unexpected fatal signal 6: dyld[8896]: symbol not found in flat
    namespace '_virQEMUCapsGet'
  error : qemuVhostUserFillDomainGPU:394 : operation failed: Unable to
    find a satisfying vhost-user-gpu

that were showing up on macOS 12 go away.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>