This adds to the "unittests" target a dependency on the
"AppTestRunApp3a" (etc.) files meant to be copied as part of the
BApplication test suite so they are generated when "jam unittests" is
run.
* TestsRules: Add "UnitTestDependency" rule.
* testapps/Jamfile: Make unit tests depend on copied files.
Fixes#12441.
Signed-off-by: Adrien Destugues <[email protected]>