Add automated tests for R embedded mode and add them to gate tasks

.gitignore

ci.hocon

com.oracle.truffle.r.test.native/Makefile

com.oracle.truffle.r.test.native/embedded/Makefile

...uffle.r.test.native/embedded/src/embedded.expected.output

...e.truffle.r.test.native/embedded/src/main.expected.output

com.oracle.truffle.r.test.native/embedded/src/main.input

com.oracle.truffle.r.test.native/embedded/test.sh

mx.fastr/copyrights/overrides

mx.fastr/mx_fastr.py