Fix #1741: sbt.ExtractAPI: extract annotations

This is necessary for correct incremental recompilation but is also used
by sbt to find tests to run (for junit they should be annotated @org.junit.Test).

I added an sbt scripted test to verify that JUnit now works, to run it:
$ sbt
> scripted discovery/test-discovery

1 files changed, 3 insertions, 0 deletions

diff --git a/sbt-bridge/sbt-test/discovery/test-discovery/src/main/scala/A.scala b/sbt-bridge/sbt-test/discovery/test-discovery/src/main/scala/A.scala
new file mode 100644
index 000000000..81f77071e
--- /dev/null
+++ b/sbt-bridge/sbt-test/discovery/test-discovery/src/main/scala/A.scala
@@ -0,0 +1,3 @@
+object A {
+  def three: Int = 4 // Hmm
+}