nix: dev workflow items (#316)

Addresses some issues making it more challenging to use the nix dev environment, including documentation and ensuring adequate space to run tests locally.

Makefile

@@ -99,14 +99,15 @@ test: ## run tests (set COMPILED_TAG_NAME to override default)
 	pytest tests/pytest --capture=no --verbose --refdir $(shell pwd)/tests/pytest/reference/circleci --image_version $(COMPILED_TAG_NAME)
 
 build_native: ## build FV3 locally (assuming all tools and dependencies are available in the environment)
-	$(MAKE) -j 8 -C FV3 GCOV=Y
+	$(MAKE) -j 8 -C FV3
 
 
 test_native: DIR=coverage_$(shell date -Is)
 test_native: ## run native tests (all tools and build dependencies are assumed to be available in the environment)
 	find FV3 -type f -name '*.gcda' -delete
 	pytest --native tests/pytest
-	pytest FV3/wrapper/tests/
+	pytest -v FV3/wrapper/tests/pytest
+	pytest -v FV3/wrapper/tests/test_all_mpi_requiring.py
 	mkdir -p $(DIR) && \
 		cd $(DIR)  && \
 		gcovr -d -r ../FV3 --html --html-details -o index.html

README.md

@@ -195,7 +195,7 @@ Then configure the build to use nix
 
     cd FV3 && ./configure nix
 
-And build the model (with coverage outputs, from the root directory)
+And build the model (from the root directory)
 
     make build_native
 
@@ -227,7 +227,8 @@ tests like this:
     # or manually
     pytest --native tests/pytest
     # and 
-    pytest FV3/wrapper/tests/
+	pytest -v FV3/wrapper/tests/pytest
+    pytest -v FV3/wrapper/tests/test_all_mpi_requiring.py
 
 When using the makefile target, code coverages reports will be saved to the
 folder `coverage_<timestamp>`.

nix/fv3/default.nix

@@ -166,6 +166,7 @@ installPhase = ''
     export PYTHONPATH=$(pwd)/tests/emulation:$(pwd)/FV3/wrapper:$PYTHONPATH
     # path to fv3.exe
     export PATH=$(pwd)/FV3:$PATH
+    export TMPDIR=/tmp
   '';
 }