gradle: test task use the output folder set by the test-reports bnd prop

The default is different than the gradle java plugin default, but using
this property provides a consistent output location for the test and
check tasks and allows the user to configure via the bnd project property
test-reports.

Fixes https://github.com/bndtools/bnd/issues/730

Signed-off-by: BJ Hargrave <bj@bjhargrave.com>