Fix lax comparison in validation tests

[WGH-]

If you add t.Logf("%+v %+v", actual, testCase.expectedErrors) to
the test code, you'll notice that only Errors' Messages are being
compared:

--- PASS: Test_ValidURLValidation/Invalid_schema (0.00s)
binding_test.go:43: [Url] [Url]

FieldNames and Classification are ignored in comparison.

Moreover, an Errors slice with a single Error with empty message
is formatted as [] (the same as empty slice), which is also
error-prone. I discovered this when working on #7791 when one test which
was not supposed to pass did pass. https://play.golang.org/p/qC4wVLrm4NG

This commit changes the test to do the comparison properly.

[codecov-io]

Codecov Report

Merging #7815 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #7815      +/-   ##
=========================================
- Coverage   41.41%   41.4%   -0.01%     
=========================================
  Files         474     474              
  Lines       63876   63876              
=========================================
- Hits        26453   26451       -2     
- Misses      33978   33980       +2     
  Partials     3445    3445

Impacted Files	Coverage Δ
models/repo_list.go	72.08% <0%> (-1.02%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update cde95f9...49f518c. Read the comment docs.

[lunny]

make L-G-T-M work