In the test header:
/* @test
...
   @library /lib/testlibrary
   @build jdk.testlibrary.OSInfo
  */

jtreg version 4.1 fcs b05:

Test results: passed: 1

jtreg version 4.1 dev b11:

Test results: error: 1

----------messages:(0/0)----------

test result: Error. No action after @build


It works only if line "@run main <Class name>" is added.
This new constraint is unexpected.