Store *.reject files in ${BUILD}/test/var/rejects/<...>/ instead of
${SOURCE}/test/<...>/.

The past approach leads to problems with testing, when the out of source
build is used and sources are on a read-only filesystem. The main
problem is when a test fails, but it is marked as fragile and should be
run again. The test fail assumes storing of the .reject file and the
testing fails on attempt to write to the read-only filesystem. The
re-run is not performed so.

Side effect for the in-source build: since the test/var/ directory is
gitignored, the *.reject files will not shown in `git status` output
anymore.

https://github.com/tarantool/test-run/pull/209

Follows up #4874