Use assert in image package unit tests

[dnephin]

Some misc cleanup

• extract a function from a rather large function in the builder
• remove setup duplication from the tests in image/
• use assert.* in the tests to make them more readable, and also for better assertions when an error is expected. Now a specific error message is expected.

[AkihiroSuda]

SGTM but windowsRS1 failing

00:07:21.216 --- FAIL: TestFSInvalidSet (0.00s)
00:07:21.217 	assert.go:107: fs_test.go:53: Expected error to contain 'is a directory', got 'rename C:\Users\ContainerAdministrator\AppData\Local\Temp\images-fs-store489919157\content\sha256\.tmp-c3ab8ff13720e8ad9047dd39466b3c8974e592c2fa383d4a3960714caef0c4f2553185424 C:\Users\ContainerAdministrator\AppData\Local\Temp\images-fs-store489919157\content\sha256\c3ab8ff13720e8ad9047dd39466b3c8974e592c2fa383d4a3960714caef0c4f2: Access is denied.'
00:07:21.217 --- FAIL: TestFSInvalidRoot (0.00s)
00:07:21.217 	assert.go:107: fs_test.go:80: Expected error to contain 'not a directory', got 'mkdir C:\Users\ContainerAdministrator\AppData\Local\Temp\images-fs-store907166895\root: The system cannot find the path specified.'
00:07:21.218 --- FAIL: TestFSMetadataGetSet (0.02s)
00:07:21.218 	assert.go:107: fs_test.go:120: Expected error to contain 'no such file or directory', got 'open C:\Users\ContainerAdministrator\AppData\Local\Temp\images-fs-store697213506\metadata\sha256\fcde2b2edba56bf408601fb721fe9b5c338d10ee429ea04fae5511b68fbf8fb9\tkey2: The system cannot find the file specified.'
00:07:21.218 --- FAIL: TestFSGetUnsetKey (0.00s)
00:07:21.218 	assert.go:107: fs_test.go:197: Expected error to contain 'no such file or directory', got 'open C:\Users\ContainerAdministrator\AppData\Local\Temp\images-fs-store803968859\content\foobar\abc: The system cannot find the path specified.'
00:07:21.219 --- FAIL: TestFSDelete (0.01s)
00:07:21.219 	assert.go:107: fs_test.go:225: Expected error to contain 'no such file or directory', got 'open C:\Users\ContainerAdministrator\AppData\Local\Temp\images-fs-store710109765\content\sha256\2c26b46b68ffc68ff99b453c1d30413413422d706483bfa0f98a5e886266e7ae: The system cannot find the file specified.'
00:07:21.219 time="2017-03-03T23:54:24Z" level=error msg="invalid image sha256:f1234d75178d892a133a410355a5a990cf75d2f33eba25d575943d4df632f3a4, invalid character 'i' looking for beginning of value" 
00:07:21.220 --- FAIL: TestRestore (0.10s)
00:07:21.220 	assert.go:107: store_test.go:43: Expected error to contain 'no such file', got 'open C:\Users\ContainerAdministrator\AppData\Local\Temp\images-fs-store684222093\metadata\sha256\f40b4ddcd4289be71c5f8f305c5b8babb4266ef2c39bc599ac968dfc6c8b720e\parent: The system cannot find the path specified.'
00:07:21.220 --- FAIL: TestAddDelete (0.02s)
00:07:21.220 	assert.go:107: store_test.go:93: Expected error to contain 'no such file or directory', got 'open C:\Users\ContainerAdministrator\AppData\Local\Temp\images-fs-store104089660\content\sha256\8d25a9c45df515f9d0fe8e4a6b1c64dd3b965a84790ddbcc7954bb9bc89eb993: The system cannot find the file specified.'
00:07:21.221 FAIL
00:07:21.221 coverage: 82.6% of statements
00:07:21.221 FAIL	github.com/docker/docker/image	0.237s

[dnephin]

I wrapped the errors and fixed the assertions, so part of the error should be consistent between platforms now.

[AkihiroSuda]

LGTM

cc @vdemeester

[vdemeester]

LGTM 🐯