fix ClientRetrieve with CAR exporting & add test coverage

[jsign]

While retrieving data with IsCAR flag enabled, the miner fails with messages no such file or directory. I extended the TestDealFlow test to cover this scenario, so it can be used to reproduce the problem, and check that it's working correctly too.

To reproduce the problem, take this PR branch, revert the client.go change and run the test.
Output:

~/code/lotus jsign/testnet3/badfiledescriptor ❯ go test ./node -run TestAPIDealFlow$  
...
...
--- FAIL: TestAPIDealFlow (6.79s)
    --- FAIL: TestAPIDealFlow/WithExportedCAR (3.31s)
        deals.go:147: open /var/folders/f1/m0gxrgsx7kg53w82gndwr4mc0000gn/T/lotus-retrieve-test-360081011/ret: no such file or directory
FAIL
FAIL    github.com/filecoin-project/lotus/node  7.312s
FAIL

Of course, with the client.go fix test pass.

The problem is the os.Open call, which opens a file in read-only mode when the main intention is writing to a file.

cc @hannahhoward

[hannahhoward]

+1 I encourage merging this -- LGTM by me

[whyrusleeping]

Thanks @jsign !