ci(java): restrict presubmit samples ITs to only snapshot (#547)

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Synth log will be available here:
https://source.cloud.google.com/results/invocations/dc44c73a-e60a-4f22-9d87-3a3f6d9d4115/targets

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Source-Link: googleapis/synthtool@27e0e91

.kokoro/build.sh

@@ -69,9 +69,16 @@ integration)
     RETURN_CODE=$?
     ;;
 samples)
-    if [[ -f samples/pom.xml ]]
+    SAMPLES_DIR=samples
+    # only run ITs in snapshot/ on presubmit PRs. run ITs in all 3 samples/ subdirectories otherwise.
+    if [[ ! -z ${KOKORO_GITHUB_PULL_REQUEST_NUMBER} ]]
     then
-        pushd samples
+      SAMPLES_DIR=samples/snapshot
+    fi
+
+    if [[ -f ${SAMPLES_DIR}/pom.xml ]]
+    then
+        pushd {SAMPLES_DIR}
         mvn -B \
           -Penable-samples \
           -DtrimStackTrace=false \

synth.metadata

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/google-oauth-java-client.git",
-        "sha": "ef4305bd8ef00890effdeb157b2f9c7929566664"
+        "sha": "fb2bec83cff3885cec73d21b814ec41e918287f8"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "b65ef07d99946d23e900ef2cc490274a16edd336"
+        "sha": "27e0e916cbfdb3d5ff6639b686cc04f78a0b0386"
       }
     }
   ],