Remove reading defaults in XSDKDefaults.cmake, test option defaults, etc.

Here I have removed the block of code that set the default from env vars CXX
and CXXFLAGS.  Just let ENABLE_LANGAUGE(CXX) do that.

I also changed the test driver to only set USE_XSDK_DEFAULTS and
XSDK_USE_COMPILER_ENV_VARS if they are set to TRUE.  This way I can test that
the defaults are being set correctly.

I removed 'NoUseCompilerEnvVars' from the test names with NoUseXsdkDefaults
because the XSDK_USE_COMPILER_ENV_VARS is never even read if
USE_XSDK_DEFAULTS=FALSE.

I fixed mispelling of 'defalt' with 'default'.
Reduce duplication