Create console test library.

This creates a TTY pair and runs `/bin/sh` in interactive mode within it.
It provides useful helper functions to interact with the shell and read the
output of commands run within it.

This is meant to be used for testing upcoming changes allowing `runsc exec` to
work in `-detach=false -tty=true` mode.

PiperOrigin-RevId: 350841006

pkg/test/testutil/BUILD

@@ -6,6 +6,7 @@ go_library(
     name = "testutil",
     testonly = 1,
     srcs = [
+        "sh.go",
         "testutil.go",
         "testutil_runfiles.go",
     ],
@@ -15,6 +16,7 @@ go_library(
         "//runsc/config",
         "//runsc/specutils",
         "@com_github_cenkalti_backoff//:go_default_library",
+        "@com_github_kr_pty//:go_default_library",
         "@com_github_opencontainers_runtime_spec//specs-go:go_default_library",
     ],
 )