[libc] Move all tests to a top level `test` directory.

A toplevel target, `check-libc` has also been added.

Reviewers: abrachet, phosek

Tags: #libc-project

Differential Revision: https://reviews.llvm.org/D72177
diff --git a/libc/docs/source_layout.rst b/libc/docs/source_layout.rst
index b06ad96..685798c 100644
--- a/libc/docs/source_layout.rst
+++ b/libc/docs/source_layout.rst
@@ -11,6 +11,7 @@
         - lib
         - loader
         - src
+        - test
         + utils
             - build_scripts
             - testing
@@ -69,6 +70,15 @@
    implementation standard document explains more about the *header*
    directories.
 
+The ``test`` directory
+----------------------
+
+This directory contains tests for the various components of llvm-libc. The
+directory structure within this directory mirrors the directory structure of the
+toplevel ``libc`` directory itself. A test for, say the ``mmap`` function, lives
+in the directory ``test/src/sys/mman/`` as implementation of ``mmap`` lives in
+``src/sys/mman``.
+
 The ``www`` directory
 ---------------------