refactor readme files: in r tests

tests/r/README.md

@@ -0,0 +1,43 @@
+# Overview of tests for the R buildpack
+
+## Tested configuration files
+
+- [`runtime.txt`](https://repo2docker.readthedocs.io/en/latest/config_files.html#runtime-txt-specifying-runtimes).
+- [`DESCRIPTION`](https://repo2docker.readthedocs.io/en/latest/config_files.html#description-install-an-r-package).
+- [`install.R`](https://repo2docker.readthedocs.io/en/latest/config_files.html#install-r-install-an-r-rstudio-environment).
+- [`requirements.txt`](https://repo2docker.readthedocs.io/en/latest/config_files.html#requirements-txt-install-a-python-environment)
+- [`apt.txt`](https://repo2docker.readthedocs.io/en/latest/config_files.html#apt-txt-install-packages-with-apt-get)
+
+## Test folders
+
+### r-rspm-apt-file
+
+- Test setup of the default R environment by omitting a version specification in
+  `runtime.txt`, where the date provided in `runtime.txt` is recent enough for a
+  RSPM snapshot of CRAN to be used.
+
+- Test use of a `apt.txt` file.
+
+### r-rspm-description-file
+
+- Test use of a `DESCRIPTION` file instead of an `install.R` file, where a
+  `runtime.txt` is omitted and a recent enough snapshot date is assumed a RSPM
+  snapshot of CRAN to be used.
+
+### r3.6-mran
+
+- Test setup of a R 3.6 environment by specifying `r-3.6-...` in `runtime.txt`,
+  where the date provided in `runtime.txt` is old enough for a MRAN snapshot of
+  CRAN to be used.
+
+### r4.0-mran
+
+- Test setup of a R 4.0 environment by specifying `r-4.0-...` in `runtime.txt`,
+  where the date provided in `runtime.txt` is old enough for a MRAN snapshot of
+  CRAN to be used.
+
+### r4.0-rspm
+
+- Test setup of a R 4.0 environment by specifying `r-4.0-...` in `runtime.txt`,
+  where the date provided in `runtime.txt` is recent enough for a RSPM snapshot
+  of CRAN to be used.

tests/r/r-rspm-apt/README.rst

@@ -1,19 +0,0 @@
-R environment - install.R
--------------------------
-
-You can install an R, RStudio, and IRKernel environment with the following
-two files:
-
-*  A ``runtime.txt`` file with the text::
-
-       r-YYYY-MM-DD
-
-   Where 'YYYY', 'MM' and 'DD' refer to a specific
-   date snapshot of https://mran.microsoft.com/timemachine
-   from which libraries will be installed.
-*  An optional ``install.R`` file that will be executed by an R installation
-   at build time. It can be used for installing packages from MRAN or GitHub.
-
-The presence of ``runtime.txt`` is enough to set up R, RStudio, and IRKernel. It
-uses the ``r-base`` package from the Ubuntu apt repositories to install
-R itself.

tests/r/r-rspm-description/README.md

@@ -1,4 +0,0 @@
-# Test that `repo2docker` will build an R package
-
-If a `DESCRIPTION` file is present along with an `R`-requiring `runtime.txt`
-, repo2docker should build the R package that is in the repository.