test: factor out the "wait" logic in bookrace

The test is currently not testing the race it is supposed to test. The
synchronisation is still valid, but needs to run at a different point.

We start with extracting the synchronisation logic for clarity.

tests/test-bookmarks-corner-case.t

@@ -120,7 +120,8 b' We build a server side extension for thi'
   > import time
   > import atexit
   > from mercurial import error, extensions, bookmarks
-  > def wrapinit(orig, self, repo):
+  > 
+  > def wait():
   >     if not os.path.exists('push-A-started'):
   >         print('setting raced push up')
   >         with open('push-A-started', 'w'):
@@ -131,6 +132,9 b' We build a server side extension for thi'
   >         if clock <= 0:
   >             raise error.Abort("race scenario timed out")
   >         time.sleep(0.1)
+  > 
+  > def wrapinit(orig, self, repo):
+  >     wait()
   >     return orig(self, repo)
   > def uisetup(ui):
   >     extensions.wrapfunction(bookmarks.bmstore, '__init__', wrapinit)