modfile: add tests for AddReplace and DropReplace

[dmitris]

PR adds a few tests for modfile's AddReplace and DropReplace functions.

[dmitris]

@rogpeppe - does the change look ok to you?

[rogpeppe]

Thanks very much for suggesting these changes. In principle I am in favour, but I also wish to keep this package as close to the original modfile package as possible. How about adding the new tests in a separate file, say replace_test.go? Then we can more easily update the rest of the source files by simply copying them.

[dmitris]

@rogpeppe - that's a good idea, I put the new tests in modfile/replace_test.go.

[rogpeppe]

LGTM, thanks!