assert,test: complete coverage for AssertionError
There were two code branches in lib/internal/assert/assertion_error.js that were not covered by tests. This adds a test to cover the two uncovered branches. The test revealed that there was some missing properties/functaionality in the AssertionError class. So this also adds the missing material.
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
commit message follows commit guidelines