Tests/CompileDefinitions: Use unique source file names for VS 6

Visual Studio 6 does not support multiple source files with the same
name in the same target even with different language extensions.  Rename
sources compiletest_mixed.(c|cpp) to compiletest_mixed_(c.c|cxx.cpp) to
avoid conflict.
Merge topic 'language-generator-expressions' into next

2c3bbae Tests/CompileDefinitions: Use unique source file names for VS 6