Fix flaky test.

Narrow down unexpected string, so it will not clash with other output.
In this case 'AutoCloseable' was printed when Gradle 7 was generating
api jar.

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/SimpleKotlinGradleIT.kt

@@ -100,7 +100,7 @@ class SimpleKotlinGradleIT : KGPBaseTest() {
         project("customJdk", gradleVersion) {
             buildAndFail("build") {
                 assertOutputContains("Unresolved reference: stream")
-                assertOutputDoesNotContain("AutoCloseable")
+                assertOutputDoesNotContain("Unresolved reference: AutoCloseable")
             }
         }
     }