Add tests for --warning

Added quotes to the test since those seem the most likely to stop
working upon future code changes.

tests/functional/export_v2/cram/warning.t

@@ -0,0 +1,38 @@
+Setup
+
+  $ source "$TESTDIR"/_setup.sh
+
+Specify a warning as text.
+
+  $ ${AUGUR} export v2 \
+  >   --tree "$TESTDIR/../data/tree.nwk" \
+  >   --warning 'A warning with "quotes"' \
+  >   --skip-validation \
+  >   --output dataset.json &>/dev/null
+
+TODO: Switch to use jq once it's available in a well-defined test environment.
+<https://github.com/nextstrain/augur/issues/1557>
+
+  $ python3 -c 'import json, sys; print(json.load(sys.stdin)["meta"]["warning"])' < dataset.json
+  A warning with "quotes"
+
+Add a warning from a markdown file.
+
+  $ cat >warning.md <<~~
+  > A warning with "quotes".
+  > ~~
+
+  $ ${AUGUR} export v2 \
+  >   --tree "$TESTDIR/../data/tree.nwk" \
+  >   --warning warning.md \
+  >   --skip-validation \
+  >   --output dataset.json &>/dev/null
+
+  $ python3 -c 'import json, sys; print(json.load(sys.stdin)["meta"]["warning"])' < dataset.json
+  A warning with "quotes".
+
+
+Note: there is an extra newline compared to the original contents. This extra
+newline is harmless and can be explained by `augur export` embedding the
+original trailing newline explicitly, followed by an additional trailing newline
+when writing to extracted-warning.md.