This patch adds destructor to ValueSerializerTest test fixture class
which reset unhandled scheduled exceptions thrown from the current test.
In some cases unhandled scheduled exceptions from current test (eg.
from one test from ValueSerializerTestWithWasm test case) produce that
Context::New(isolate()) from next test's constructor returns NULL.

This error is easily reproduced when unittest file (compiled for ARM or
MIPS simulator) is directly executed (not by tools/run_tests.py script),
so all tests are executed together. When the script is used, each test
from ValueSerializerTestWithWasm test case is separately executed and
unhandled exception from one test doesn't efect to another test from
the same test case.

BUG=

Review-Url: https://codereview.chromium.org/2644083002
Cr-Commit-Position: refs/heads/master@{#42547}