Clean up exit codes in tests

After JDK-8222111 is fixed, there are two follow-ups:
 
First, the exit code should be communicated with "return $code;" rather than "exit($code)" in some tests. For example:
 test/jdk/java/lang/reflect/exeCallerAccessTest/exeCallerAccessTest.c
 test/jdk/tools/launcher/exeJniInvocationTest.c

Second, the returns from "int main(...)" should always be present, for example in:
  test/jdk/tools/launcher/exeJniInvocationTest.c