tests: shell: Fix for ineffective 0007-mid-restore-flush_0

tests: shell: Fix for ineffective 0007-mid-restore-flush_0

The test did not catch non-zero exit status of the spawned coprocess. To
make it happen, Drop the line killing it (it will exit anyway) and pass
its PID to 'wait'.

While being at it, put the sleep into the correct spot (otherwise the
check for chain 'foo' existence fails as it runs too early) and make
said chain existence check effective.

Fixes: 4e3c11a6f5a94 ("nft: Fix for ruleset flush while restoring")
Signed-off-by: Phil Sutter <phil@nwl.cc>