Allow expect_snapshot_value() to ignore attributes

[DanChaltiel]

Attributes are often of lesser importance and checking on them unnecessarily clutters the output.

For instance, expect_snapshot_value(x, ignore_attributes=TRUE) would be very nice to use.

Also, this could sometimes allow using nicer styles, as shown in #1771.

[hadley]

Isn't ... already passed on to waldo::compare()? That's certainly what the docs and code suggests to me.