Modify our internal release script to run Gradle integration tests during Guava releases.

And make that process a little faster and more hygienic.

(I see no reason to expect us to [get the Gradle setup wrong](#6612) again, but it would be quite embarrassing if we did....)

RELNOTES=n/a
PiperOrigin-RevId: 544680176

Author: cpovirk

util/gradle_integration_tests.sh

@@ -2,10 +2,35 @@
 
 set -eu
 
-mvn clean install -DskipTests
-mvn clean install -DskipTests -f android
+mvn clean install --projects '!guava-testlib,!guava-tests,!guava-bom,!guava-gwt' -Dmaven.test.skip=true -Dmaven.javadoc.skip=true
+mvn clean install --projects '!guava-testlib,!guava-tests,!guava-bom' -Dmaven.test.skip=true -Dmaven.javadoc.skip=true -f android
 
-integration-tests/gradle/gradlew -p integration-tests/gradle wrapper --gradle-version=5.6.4
-integration-tests/gradle/gradlew -p integration-tests/gradle testClasspath
-integration-tests/gradle/gradlew -p integration-tests/gradle wrapper --gradle-version=7.0.2
-integration-tests/gradle/gradlew -p integration-tests/gradle testClasspath
+# Gradle Wrapper overwrites some files when it runs.
+# To avoid modifying the Git client, we copy everything we need to another directory.
+# That provides general hygiene, including avoiding release errors:
+#
+# Preparing to update Javadoc and JDiff for the release...
+# error: Your local changes to the following files would be overwritten by checkout:
+#         integration-tests/gradle/gradle/wrapper/gradle-wrapper.jar
+#         integration-tests/gradle/gradle/wrapper/gradle-wrapper.properties
+#         integration-tests/gradle/gradlew
+#         integration-tests/gradle/gradlew.bat
+# Please commit your changes or stash them before you switch branches.
+
+GRADLE_TEMP="$(mktemp -d)"
+trap 'rm -rf "${GRADLE_TEMP}"' EXIT
+
+# The Gradle tests need the pom.xml only to read its version number.
+# (And the file needs to be two directory levels up from the Gradle build file.)
+# TODO(cpovirk): Find a better way to give them that information.
+cp pom.xml "${GRADLE_TEMP}"
+
+for version in 5.6.4 7.0.2; do
+  # Enter a subshell so that we return to the current directory afterward.
+  (
+    cp -r integration-tests "${GRADLE_TEMP}/${version}"
+    cd "${GRADLE_TEMP}/${version}/gradle"
+    ./gradlew wrapper --gradle-version="${version}"
+    ./gradlew testClasspath
+  )
+done