unittests: avoid deprecated size_t property indexing.

9 jobs for type-correctness in 4 minutes and 6 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #67284
VS2015
VS2015-32-bit

00:01:09

passed #67283
VS2015
VS2015-64-bit

00:01:22

passed #67282
VS2017
VS2017-32-bit

00:01:11

passed #67281
VS2015
VS2017-64-bit

00:01:27

passed #67278
Linux
clang-c++11

00:03:05

passed #67280
Linux
cppcheck

00:00:49

passed #67285
Linux
doc

00:00:21

passed #67277
Linux
gcc-c++11

00:03:57

passed #67279
Apple
macos-c++11

00:01:40