Fix several leaks in tests

[dylanahsmith]

In order to address #222, we need to avoid leaks in the tests as well as the code under test, since we currently depend on code using the library to dispose isolates and close contexts to avoid memory leaks.

This PR just adds the missing Isolate.Dispose and Context.Close calls that came up from using clang's address sanitizer.

[codecov]

Codecov Report

Merging #225 (9e708a4) into master (24e3ca7) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #225   +/-   ##
=======================================
  Coverage   95.59%   95.59%           
=======================================
  Files          16       16           
  Lines         545      545           
=======================================
  Hits          521      521           
  Misses         15       15           
  Partials        9        9           

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