squash! Add support for tap output from runtime test

Granular TAP output makes it easier to see what's being tested (and
what's going wrong).  You could feed this into any TAP harness you
like, but I've chosen prove (it seems popular).  You need a TAP
harness looking for test failures, because runtimetest now returns
zero when it successfully runs the tests (regardless of whether the
tests all pass).

The OPTIND shift is out of getopts(1p).

I've dropped the "VALIDATING RUNTIME" header since prove displays the
command it's running in its usual output (e.g. "/usr/bin/runc .. ok").

Author: wking

README.md

@@ -29,23 +29,38 @@ INFO[0000] Bundle validation succeeded.
 
 ## Testing OCI runtimes
 
-```sh
+With [prove][] installed:
+
+```
 $ make
 $ sudo make install
-$ sudo ./test_runtime.sh -r runc
------------------------------------------------------------------------------------
-VALIDATING RUNTIME: runc
------------------------------------------------------------------------------------
-validating container process
-validating capabilities
-validating hostname
-validating rlimits
-validating sysctls
-Runtime runc passed validation
+$ sudo ./test_runtime.sh -r /usr/bin/runc
+/usr/bin/runc .. ok
+All tests successful.
+Files=1, Tests=90,  0 wallclock secs ( 0.02 usr  0.01 sys +  0.02 cusr  0.01 csys =  0.06 CPU)
+Result: PASS
+```
+
+You can also use the usual prove options (separated by `--`):
+
+```
+$ sudo ./test_runtime.sh -r /usr/bin/runc -- -v
+/home/wking/bin/runc ..
+TAP version 13
+ok 1 - # SKIP root.readonly falsy
+ok 2 - hostname matches expected value
+…
+ok 90 - # SKIP linux.gidMappings checks (no mappings specified)
+1..90
+ok
+All tests successful.
+Files=1, Tests=90,  0 wallclock secs ( 0.02 usr  0.00 sys +  0.02 cusr  0.01 csys =  0.05 CPU)
+Result: PASS
 ```
 
 [bundle]: https://github.com/opencontainers/runtime-spec/blob/master/bundle.md
 [config.json]: https://github.com/opencontainers/runtime-spec/blob/master/config.md
+[prove]: http://perldoc.perl.org/prove.html
 [runC]: https://github.com/opencontainers/runc
 [runtime-spec]: https://github.com/opencontainers/runtime-spec