STYLE: Add missing source file extension to `Common` module tests
STYLE: Add missing source file extension to `Common` module tests

Add missing source file extension to `Common` module tests.

Inadvertently missed in commits 244567b and 1e3df39.

Fixes:
```
CMake Warning (dev) at CMake/ITKModuleTest.cmake:62 (add_executable):
  Policy CMP0115 is not set: Source file extensions must be explicit.  Run
  "cmake --help-policy CMP0115" for policy details.  Use the
  cmake_policy
  command to set the policy and suppress this warning.

  File:

  /Users/builder/external/ITK/Modules/Core/Common/test/itkRealTimeIntervalTest.cxx
Call Stack (most recent call first):
  Modules/Core/Common/test/CMakeLists.txt:192 (CreateTestDriver)
This warning is for project developers.  Use -Wno-dev to suppress it.
```
for all three test files:
```
itkRealTimeIntervalTest.cxx
itkRealTimeStampTest.cxx
itkMultiThreaderTypeFromEnvironmentTest.cxx
```

raised in:
https://open.cdash.org/build/7079554/configure

Kitware/ITK,thewtex/ITK,richardbeare/ITK,Kitware/ITK,vfonov/ITK,vfonov/ITK,InsightSoftwareConsortium/ITK,thewtex/ITK,hjmjohnson/ITK,vfonov/ITK,InsightSoftwareConsortium/ITK,richardbeare/ITK,BRAINSia/ITK,BRAINSia/ITK,BRAINSia/ITK,thewtex/ITK,Kitware/ITK,thewtex/ITK,thewtex/ITK,LucasGandel/ITK,Kitware/ITK,LucasGandel/ITK,Kitware/ITK,vfonov/ITK,BRAINSia/ITK,InsightSoftwareConsortium/ITK,vfonov/ITK,hjmjohnson/ITK,InsightSoftwareConsortium/ITK,hjmjohnson/ITK,richardbeare/ITK,thewtex/ITK,hjmjohnson/ITK,BRAINSia/ITK,richardbeare/ITK,BRAINSia/ITK,InsightSoftwareConsortium/ITK,LucasGandel/ITK,Kitware/ITK,richardbeare/ITK,hjmjohnson/ITK,thewtex/ITK,richardbeare/ITK,vfonov/ITK,vfonov/ITK,LucasGandel/ITK,richardbeare/ITK,BRAINSia/ITK,vfonov/ITK,InsightSoftwareConsortium/ITK,Kitware/ITK,LucasGandel/ITK,LucasGandel/ITK,hjmjohnson/ITK,hjmjohnson/ITK,InsightSoftwareConsortium/ITK,LucasGandel/ITK,vfonov/ITK,LucasGandel/ITK