Fix undeterministic call to shouldPoll() in Avalanche tests
Fix undeterministic call to shouldPoll() in Avalanche tests

Summary:
These tests pass consistently with `make check` but fail `ninja check` consistently with the following setup:
```
gcc 8.1.0
clang 7.0.1
boost 1.58.0
```
The failure occurs at step `i = 9`, where `registerPoll()` is called, setting inflight to `10`
and then immediately after, the call to `shouldPoll()` returns `false`.

Based on reading of the test, it seems clear that shouldPoll() should be evaluated before registerPoll() is called, but apparently compiler differences evaluate the BOOST_CHECK_EQUAL macro differently.

Before writing this diff, I attempted to write the test in such a way as to evaluate registerPoll() and shouldPoll(), in that order, in intermediate variables, as this is the expected order of evaluation when reading the BOOST_CHECK_EQUAL call. This test fails for ninja and make as expected:
```
-        BOOST_CHECK_EQUAL(vrinflight.registerPoll(), vrinflight.shouldPoll());
+        bool rp = vrinflight.registerPoll();
+        bool sp = vrinflight.shouldPoll();
+        BOOST_CHECK_EQUAL(rp, sp);
```

Test Plan:
```
make check
ninja check
```

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2504

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