Clean up JNI_ENV and JVMTI_ENV macros

A lot tests are trying to serve C and C++ and therefore do things like:

#ifndef JNI_ENV_ARG

#ifdef __cplusplus
#define JNI_ENV_ARG(x, y) y
#define JNI_ENV_PTR(x) x
#else
#define JNI_ENV_ARG(x,y) x, y
#define JNI_ENV_PTR(x) (*x)
#endif

#endif

This leads to less-than-ideal readable code such as:
return JNI_ENV_PTR(env)->ThrowNew(JNI_ENV_ARG(env, exc_class), msg);

We can clean up these macros since now cpp or c test files are compiled by one or the other languages and this will help readability of the actual tests.

Other macros to clean up:
  JVMTI_ENV*

And remove the #ifdef __cplusplus parts.