Update the opam invocation

doc/manual.org

@@ -1184,7 +1184,7 @@ You should set the =build:= field of your =<package>.opam= file as
 follows:
 
 #+begin_src
-build: [["jbuilder" "build" "-p" "<package>" "-j" jobs]]
+build: [["jbuilder" "build" "-p" name "-j" jobs]]
 #+end_src
 
 =-p pkg= is a shorthand for =--root . --only-packages pkg=. =-p= is
@@ -1192,10 +1192,23 @@ the short version of =--for-release-of-packages=.
 
 This has the following effects:
 - it tells jbuilder to build everything that is installable and to
-  ignore packages other than =<package>= defined in your project
+  ignore packages other than =name= defined in your project
 - it sets the root to prevent jbuilder from looking it up
 - it uses whatever concurrency option opam provides
 
+Note that =name= and =jobs= are variables expanded by opam. =name=
+expands to the package name and =jobs= to the number of jobs available
+to build the package.
+
+*** Tests
+
+To setup the building and running of tests in opam, add this line to
+your =<package>.opam= file:
+
+#+begin_src
+build-test: [["jbuilder" "runtest" "-p" name "-j" jobs]]
+#+end_src
+
 ** Workspace configuration
 
 By default, a workspace has only one build context named =default=