Add --test flag to jpm.

Also test some select packages when testing installation.
This is used in CI to make sure that versions of Janet work with
the most libraries.

jpm.1

@@ -28,6 +28,10 @@ More interesting are the local commands. For more information on jpm usage, see
 .BR \-\-verbose
 Print detailed messages of what jpm is doing, including compilation commands and other shell commands.
 
+.TP
+.BR \-\-test
+If passed to jpm install, runs tests before installing. Will run tests recursively on dependencies.
+
 .SH OPTIONS
 
 .TP