[JS] enable integration tests for MapVector

[asfimport]

As of 0.14, C++ and Java support Map arrays those implementations pass integration tests. JS has a MapVector and some unit tests for it, but it should be tested against other implementations as well

Reporter: Ben Kietzman / @bkietz

Related issues:

• [Integration][JS] Integration Tests for Map Type (duplicates)

_{Note: This issue was originally created as ARROW-5763. Please see the migration documentation for further details.}

[asfimport]

Paul Taylor / @trxcllnt:
After reviewing the C++, the JS version of the Map type is not the same (it's essentially a Struct except instead of accessing child fields by field index, they're accessed by name). We should absolutely update the JS Map implementation before the 1.0 release.