Build: Add bwcTest target to ci script (elastic/x-pack-elasticsearch#1488)

This is to allow bwcTest to be run by ci. This commit also fixes the
existing packagingTest target, which was never updated after the xpack
repo split.

Original commit: elastic/x-pack-elasticsearch@f4696c1899

dev-tools/ci

@@ -29,7 +29,13 @@ case $key in
         GRADLE_CLI_ARGS=(
             "--info"
             "-Pvagrant.boxes=all"
-            ":x-pack:qa:vagrant:packagingTest"
+            "packagingTest"
+        )
+        ;;
+    bwcTest)
+        GRADLE_CLI_ARGS=(
+            "--info"
+            "bwcTest"
         )
         ;;
     check)