adb: try harder to fill our test sockets.

On platforms that implement sockets via underlying sockets (e.g. Wine),
a socket can appear to be full, and then become available for writes
again without read being called on the other end. Add a sleep after
each write to give the underlying implementation time to flush.

This doesn't help us if the buffer size is smaller than MAX_PAYLOAD,
but at least in the case of Wine, that doesn't seem to be the case.

Test: adb_test
Test: wine adb_test.exe
Change-Id: Iff1ec14bbf318b9742ec7e2fb72e34e3d6bbe6ad