Try to make ConfigurationPropertySourcesTests perf tests more robust

Previously, the tests used absolute values to verify that the work
had completed sufficiently quickly. This led to flaky tests in
environments where the performance can be variable such as CI.

This commit tries to make the tests more robust by comparing the
performance to a baseline and requiring it to be twice as fast.

Closes gh-22137

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/context/properties/source/ConfigurationPropertySourcesTests.java

@@ -127,20 +127,24 @@ class ConfigurationPropertySourcesTests {
 
 	@Test // gh-20625
 	void environmentPropertyAccessWhenImmutableShouldBePerformant() {
-		testPropertySourcePerformance(true, 1000);
+		long baseline = testPropertySourcePerformance(false);
+		long immutable = testPropertySourcePerformance(true);
+		assertThat(immutable).isLessThan(baseline / 2);
 	}
 
 	@Test // gh-20625
 	void environmentPropertyAccessWhenMutableWithCacheShouldBePerformant() {
 		StandardEnvironment environment = createPerformanceTestEnvironment(false);
+		long uncached = testPropertySourcePerformance(environment);
 		ConfigurationPropertyCaching.get(environment).enable();
-		testPropertySourcePerformance(environment, 1000);
+		long cached = testPropertySourcePerformance(environment);
+		assertThat(cached).isLessThan(uncached / 2);
 	}
 
 	@Test // gh-20625
 	@Disabled("for manual testing")
 	void environmentPropertyAccessWhenMutableShouldBeTolerable() {
-		testPropertySourcePerformance(false, 5000);
+		assertThat(testPropertySourcePerformance(false)).isLessThan(5000);
 	}
 
 	@Test // gh-21416
@@ -158,9 +162,9 @@ class ConfigurationPropertySourcesTests {
 		assertThat(total).isLessThan(1000);
 	}
 
-	private void testPropertySourcePerformance(boolean immutable, int maxTime) {
+	private long testPropertySourcePerformance(boolean immutable) {
 		StandardEnvironment environment = createPerformanceTestEnvironment(immutable);
-		testPropertySourcePerformance(environment, maxTime);
+		return testPropertySourcePerformance(environment);
 	}
 
 	private StandardEnvironment createPerformanceTestEnvironment(boolean immutable) {
@@ -173,14 +177,14 @@ class ConfigurationPropertySourcesTests {
 		return environment;
 	}
 
-	private void testPropertySourcePerformance(StandardEnvironment environment, int maxTime) {
+	private long testPropertySourcePerformance(StandardEnvironment environment) {
 		long start = System.nanoTime();
 		for (int i = 0; i < 1000; i++) {
 			environment.getProperty("missing" + i);
 		}
 		long total = TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - start);
 		assertThat(environment.getProperty("test-10-property-80")).isEqualTo("test-10-property-80-value");
-		assertThat(total).isLessThan(maxTime);
+		return total;
 	}
 
 	static class TestPropertySource extends MapPropertySource implements OriginLookup<String> {