test: add README

Updates #21034

Change-Id: I951fb48ab3b9ed54d225c11879db8f09048a36a3
Reviewed-on: https://go-review.googlesource.com/50950Reviewed-by: Rob Pike <r@golang.org>

test/README.md

+The test directory contains tests of the Go tool chain and runtime.
+It includes black box tests, regression tests, and error output tests.
+They are run as part of all.bash.
+
+To run just these tests, execute:
+
+    go run run.go
+
+Standard library tests should be written as regular Go tests in the appropriate package.
+
+The tool chain and runtime also have regular Go tests in their packages.
+The main reasons to add a new test to this directory are:
+
+* it is most naturally expressed using the test runner; or
+* it is also applicable to `gccgo` and other Go tool chains.