Add tests for r-in-conda

tests/conda/r/environment.yml

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+dependencies:
+  - r-base=3.6
+  - r-ggplot2

tests/conda/r/verify

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+#!/bin/sh
+
+jupyter serverextension list 2>&1 | grep jupyter_server_proxy
+jupyter nbextension list 2>&1 | grep jupyter_server_proxy
+R -e "library('ggplot2')"