testing: allow negative indexes for valid/invalid

Author: brettz9

CONTRIBUTING.md

@@ -58,7 +58,8 @@ flag passed to `test-index`, e.g., for `check-examples` and `require-example`:
 `npm run --rule=check-examples,require-example test-index`.
 
 You can further limit this by providing `--invalid` and/or `--valid` flags
-with a comma-separated list of 0-based indexes that you wish to include. For
+with a comma-separated list of 0-based indexes that you wish to include (also
+accepts negative offsets from the end, e.g., `-1` for the last item). For
 example, to check the first and third invalid tests of `check-examples`
 alon with the second valid test, you can run:
 

test/rules/index.js

@@ -58,7 +58,8 @@ const ruleTester = new RuleTester();
   if (process.env.npm_config_invalid) {
     const indexes = process.env.npm_config_invalid.split(',');
     assertions.invalid = assertions.invalid.filter((assertion, idx) => {
-      return indexes.includes(String(idx));
+      return indexes.includes(String(idx)) ||
+        indexes.includes(String(idx - assertions.invalid.length));
     });
     if (!process.env.npm_config_valid) {
       assertions.valid = [];
@@ -67,7 +68,8 @@ const ruleTester = new RuleTester();
   if (process.env.npm_config_valid) {
     const indexes = process.env.npm_config_valid.split(',');
     assertions.valid = assertions.valid.filter((assertion, idx) => {
-      return indexes.includes(String(idx));
+      return indexes.includes(String(idx)) ||
+        indexes.includes(String(idx - assertions.valid.length));
     });
     if (!process.env.npm_config_invalid) {
       assertions.invalid = [];