tests: fix rare false failure in tail-2/F-vs-rename

tests: fix rare false failure in tail-2/F-vs-rename

This is wrong fix really, as only introducing delay I think.

* tests/tail-2/F-vs-rename.sh: Avoid a rare false failure
due to a race in the test.  Now wait until tail has noticed
that b is replaced before writing to a, so that the subsequent
write of "y" to b will be displayed independently from
current contents of b ("x").