Build tags ignored in test files

[rclark]

I am seeing that when an example function is defined in a _test.go file has some build tag constraint, those constraints don't seem to be respected by gomarkdoc.

I put together a little repo to demonstrate this. One func Example() is defined in a file that should only be selected when the build tag example is on, another func Example() is in a file that should only be selected when the build tag is off. But both examples show up when I run gomarkdoc, regardless of whether I specify any build tags in GOFLAGS or --tags.