Support extracting code when initializing test data without field name

[meoconbatu]

This commit only supports anonymous struct type. To fully support for named struct type requires a lot of changes in the source code.

Fixes #78

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[coveralls]

Pull Request Test Coverage Report for Build 5382124734

• 20 of 22 (90.91%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.9%) to 65.835%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
testrunner/ast.go	20	22	90.91%
<!--	Total:	20	22	90.91%	-->

Files with Coverage Reduction	New Missed Lines	%
testrunner/ast.go	1	75.93%
<!--	Total:	1		-->

Totals
Change from base Build 4500252823:	0.9%
Covered Lines:	422
Relevant Lines:	641

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💛 - Coveralls