abstract explicit schema version specifiers in tests for easier maintenance upon schema updates

[zacharyburnett]

asdf/asdf/_tests/test_history.py

Line 105 in a8f4769

ff = asdf.AsdfFile(version="1.6.0")

should these references to 1.6.0 be abstracted to some central place for easy updating?

Originally posted by @zacharyburnett in #1962 (comment)

I'm in favor of it.

For this test it's already assuming 1.6.0 (see line 111) and the change here is one way to address the failure of this test with 1.7.0 (an alternative would be to update 111 to check for 1.7.0 when that's the default version). The other test updates in this PR are similar (for example expecting a specific ndarray tag that only exists in 1.6.0).

Originally posted by @braingram in #1962 (comment)