Fix flaky test ExternalSSTFileBasicTest.Basic

[jowlyzhang]

This test is flaky likely due to synchronization of the file ingestion thread and the live write thread with test sync points are not working as expected sometimes. Very occasionally, the live write thread can enter the write queue after file ingestion job already dequeued. Or it entered and waited for a very short period of time and quickly returned in the fast path:

rocksdb/db/write_thread.cc

Lines 83 to 86 in 833a226

	// This is below the fast path, so that the stat is zero when all writes are
	// from the same thread.
	PERF_TIMER_FOR_WAIT_GUARD(write_thread_wait_nanos);

To fix the flakiness, I moved the test sync points to make sure the write thread is already linked into the write queue before the file ingestion writer get dequeued, so it definitely would need to wait some time in order to do its write.

Test plan:
I'm able to reproduce the flakiness with this command before the fix with every two or three runs:
./gtest-parallel external_sst_file_basic_test --gtest_filter=ExternalSSTFileBasicTest.Basic --repeat=10000 --workers=100

After the fix, I have tried the command for 10 runs, and there is no failure detected.