[libc][docs] Document libc-shared-tests ninja target (#189062)

Added a brief description of the libc-shared-tests target to the
Building and Testing page.

This target allows running tests for shared standalone components like
math primitives without the full libc runtime.

libc/docs/build_and_test.rst

@@ -38,6 +38,12 @@ The libc can be built and tested in two different modes:
 
         $> ninja libc-integration-tests
 
+   #. Shared tests - You can run tests for shared, standalone components (like math primitives) without needing the full libc runtime by the command:
+
+      .. code-block:: sh
+
+        $> ninja libc-shared-tests
+
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