Fix flaky EagerReaderTest.test_large_data_multibyte

eager_reader_read() makes no guarantee that the number of bytes read
equals the number of bytes to be read. Insisting that it must in the
test can lead to race conditions where not all data is read at once and
the test fails.

Adjusted test to match API promise.

Change-Id: I5df4b6d8df61d0601402511bb3d9f29f22378981