doc: document test command

Add documentation for the test command, including the newly added =~
operator and some gotchas wrt. the numeric comparisons.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Rasmus Villemoes <ravi@prevas.dk>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index 372ef56..c5b45fd 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -123,6 +123,7 @@
    cmd/source
    cmd/tcpm
    cmd/temperature
+   cmd/test
    cmd/tftpput
    cmd/trace
    cmd/true