Produce test report for integration tests (#2600)

BATS supports outputing test-reports in various formats (including the
format used by our Java/JUnit tests), but our build wasn't taking
advantage of this.

This commit tweaks our BATS invocation so that a JUnit-style XML test
report ('report.xml') is produced in the packaging module's
'build/test-output' directory.

Author: gerlowskija

solr/packaging/build.gradle

@@ -294,7 +294,7 @@ class BatsTask extends Exec {
 
     // Note: tests to run must be listed after all other arguments
     // Additional debugging output: -x, --verbose-run
-    setArgs(['-T', '--print-output-on-failure'] + (testFiles.empty ? testDir : testFiles))
+    setArgs(['-T', '--print-output-on-failure', '--report-formatter', 'junit', '--output', "$project.buildDir/test-output"] + (testFiles.empty ? testDir : testFiles))
 
 
     super.exec()