test: add cases for testing imageCache.get

[faycheng]

Signed-off-by: 程飞 [email protected]

Ⅰ. Describe what this PR did

add cases for testing imageCache.get

Ⅱ. Does this pull request fix one issue?

Ⅲ. Describe how you did it

this pr is related to issue #975 & pr #973 .please don't merge it before merging #973

Ⅳ. Describe how to verify it

make unit-test

Ⅴ. Special notes for reviews

[HusterWan]

@faycheng if working your pr in process, you can add prefix [WIP] in your pr title. Thanks a lot.

[faycheng]

@HusterWan Done.

[YaoZengzeng]

@faycheng please rebase your PR, thanks :)

[faycheng]

@YaoZengzeng Done.
And in order to trigger travis ci again, i will mark this pr closed and then mark it opened.

Travis log:

> sudo apt-get update -qq
W: The repository 'http://www.apache.org/dist/cassandra/debian 39x Release' does not have a Release file.
W: http://ppa.launchpad.net/couchdb/stable/ubuntu/dists/trusty/Release.gpg: Signature by key 15866BAFD9BCC4F3C1E0DFC7D69548E1C17EAB57 uses weak digest algorithm (SHA1)
E: Failed to fetch http://dl.bintray.com/apache/cassandra/dists/39x/main/binary-amd64/Packages  502  Bad Gateway [IP: 52.27.175.225 80]
E: Failed to fetch http://dl.bintray.com/apache/cassandra/dists/39x/main/binary-i386/Packages  502  Bad Gateway [IP: 52.27.175.225 80]
E: Some index files failed to download. They have been ignored, or old ones used instead.
The command "sudo apt-get update -qq" failed and exited with 100 during .

[codecov-io]

Codecov Report

Merging #979 into master will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #979      +/-   ##
==========================================
+ Coverage   13.04%   13.09%   +0.04%     
==========================================
  Files         124      124              
  Lines        8354     8354              
==========================================
+ Hits         1090     1094       +4     
+ Misses       7171     7168       -3     
+ Partials       93       92       -1

Impacted Files	Coverage Δ
daemon/mgr/image.go	32.12% <0%> (+2.07%)	⬆️

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[faycheng]

Remove the WIP label

[YaoZengzeng]

LGTM