Write tests for handling /near/ links

[gnprice]

This is a follow-up to #82. For the launch we'll be shipping an implementation (from #1566) that doesn't yet have tests, in the interest of timeliness. It'll be good to follow up by writing those tests.

This issue is for one swath of those tests, corresponding to a segment of the commits. Tests for the other commits are tracked in:

• Write tests for msglist widgets handling middle of history #1569

To see which tests are needed, grep for comments with TODO(#1570).

Here's also a description of the tests needed, in terms of a draft version of #1566.

a8fd383 wip internal_link [nfc]: Propagate InternalLink up to caller; TODO test?

(I ended up adding the couple of checks this called for in a revision of #1566.)

lib/model/internal_link.dart | 14 +++++++++-----
lib/widgets/content.dart | 18 ++++++++++--------
test/model/internal_link_test.dart | 11 ++++++++++-
3 files changed, 29 insertions(+), 14 deletions(-)

251dab4 wip internal_link: Parse /near/ in narrow links; TODO test

lib/model/internal_link.dart | 11 +++++++----
1 file changed, 7 insertions(+), 4 deletions(-)

8c1ba1f wip/link2 internal_link: Open /near/ links at the given anchor in msglist; TODO test

lib/notifications/display.dart | 2 +-
lib/widgets/content.dart | 3 ++-
2 files changed, 3 insertions(+), 2 deletions(-)