feat: add bats tests, docs (#139)

* fix(test): ensure bats file list uses xargs

* docs(readme): use yarn for testing instructions

* fix(test): ensure pipelines fail properly

* docs(alias): fix table header

---------

Signed-off-by: Ismo Vuorinen <ismo@ivuorinen.net>

local/bin/x-until-success.md

@@ -0,0 +1,20 @@
+# x-until-success
+
+Repeat a command until it succeeds (exit status 0). The command is
+always executed at least once.
+
+## Usage
+
+```bash
+x-until-success [--sleep SECONDS] command [args...]
+```
+
+Use `--sleep` to control the delay between attempts.
+
+### Example
+
+```bash
+x-until-success --sleep 5 curl -I https://example.com
+```
+
+<!-- vim: set ft=markdown spell spelllang=en_us cc=80 : -->