Re-enable testing the book
Re-enable testing the book

In #29932, I moved the location of TRPL, but I missed making the changes
in mk/tests.mk. This led to #30088 landing with a broken example.

As such, #30113 will need to land before this.

GBGamer/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,GBGamer/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust