add a testcase for successful create but failed flush
add a testcase for successful create but failed flush

described in Goofys.FlushFile:

// if we returned success from creat() earlier
// linux may think this file exists even when it doesn't
// TODO: figure out a way to make the kernel forget this inode

I can reproduce this manually but not in the tests here. Disabled the
tests if we are testing with s3proxy due to
https://github.com/andrewgaul/s3proxy/issues/182

kahing/goofys,kahing/goofys,kahing/goofys