Test executable crashes when run under `Valgrind`

Output from `Valgrind` regarding the illegal instruction that led to the crash.

```
2019-12-13 13:57:59,742307 UTC [4656] DEBUG CTestObserver.cc@23 +-----------------------------------------------------+
2019-12-13 13:57:59,742946 UTC [4656] DEBUG CTestObserver.cc@24 | CJsonOutputStreamWrapperTest/testConcurrentWrites |
2019-12-13 13:57:59,743548 UTC [4656] DEBUG CTestObserver.cc@25 +-----------------------------------------------------+
vex amd64->IR: unhandled instruction bytes: 0x66 0xF 0x3A 0x63 0x0 0x10 0x83 0xF9
vex amd64->IR: REX=0 REX.W=0 REX.R=0 REX.X=0 REX.B=0
vex amd64->IR: VEX=0 VEX.L=0 VEX.nVVVV=0x0 ESC=0F3A
vex amd64->IR: PFX.66=1 PFX.F2=0 PFX.F3=0
==4656== valgrind: Unrecognised instruction at address 0x23dd20.
==4656== Your program just tried to execute an instruction that Valgrind
==4656== did not recognise. There are two possible reasons for this.
==4656== 1. Your program has a bug and erroneously jumped to a non-code
==4656== location. If you are running Memcheck and you just saw a
==4656== warning about a bad jump, it's probably your program's fault.
==4656== 2. The instruction is legitimate but Valgrind doesn't handle it,
==4656== i.e. it's Valgrind's fault. If you think this is the case or
==4656== you are not sure, please let us know and we'll try to fix it.
==4656== Either way, Valgrind will now raise a SIGILL signal which will
==4656== probably kill your program.
si_signo 4, si_code: 1, si_errno: 0, address: 0x23dd20, library: ./ml_test, base: 0x108000, normalized address: 0x135d20
make[3]: *** [memcheck] Illegal instruction
make[3]: Leaving directory `/home/eds/src/ml-cpp/lib/core/unittest'
/home/eds/src/ml-cpp/lib/core make test FAILURE!!!
make[2]: *** [memcheck] Error 1
make[2]: Leaving directory `/home/eds/src/ml-cpp/lib/core'
make[1]: *** [memcheck] Error 1
make[1]: Leaving directory `/home/eds/src/ml-cpp/lib'
make: *** [memcheck] Error 1

real 64m10.560s
user 63m40.264s
sys 0m19.411s
```