fix(grails-gradle): skip missing input dirs in mergeTestReports

The TestReport task registered by TestPhasesGradlePlugin reads test
results from 'test-results/<phase>/binary' directories. When a phase
is skipped (e.g. integrationTest under -PonlyCoreTests) the directory
does not exist and the TestReport task fails with 'Could not write
test report for results in [...]'.

Filter testResults with { File f -> f.exists() } so non-existent
directories are silently dropped. When nothing is produced for a
phase, the merge report is just smaller; when all phases skip, the
merge report is empty but the task still succeeds.

Fixes 14 cascade failures in grails-test-examples-* modules when the
build is invoked with -PonlyCoreTests (the flag used by the CI
'Build Grails-Core' job).

Author: jamesfredley

grails-gradle/plugins/src/main/groovy/org/grails/gradle/plugin/core/TestPhasesGradlePlugin.groovy

@@ -142,15 +142,15 @@ class TestPhasesGradlePlugin implements Plugin<Project> {
             it.mustRunAfter(project.tasks.withType(Test).toArray())
             it.destinationDirectory.set(project.layout.buildDirectory.dir('reports/tests'))
             it.testResults.from(
-                    project.files(project.layout.buildDirectory.dir('test-results/test/binary'))
+                    project.files(project.layout.buildDirectory.dir('test-results/test/binary')).filter { File f -> f.exists() }
             )
         }
     }
 
     private static void addPhaseToMergeTestReports(Project project, String phaseName) {
         project.tasks.named(MERGE_TEST_REPORTS_TASK_NAME, TestReport) {
             it.testResults.from(
-                    project.files(project.layout.buildDirectory.dir("test-results/${phaseName}/binary"))
+                    project.files(project.layout.buildDirectory.dir("test-results/${phaseName}/binary")).filter { File f -> f.exists() }
             )
         }
     }