Add support for property file encoding in @TestPropertySource

Prior to this commit, property files configured (or inferred) via
@TestPropertySource were read using the default encoding of the JVM.

This commit introduces an `encoding` attribute in @TestPropertySource
which allows developers to specify an explicit encoding for test
property files.

Closes gh-30982

spring-test/src/test/java/org/springframework/test/context/env/CustomEncodingTestPropertySourceTests.java

@@ -0,0 +1,70 @@
+/*
+ * Copyright 2002-2023 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.test.context.env;
+
+import org.junit.jupiter.api.Nested;
+import org.junit.jupiter.api.Test;
+
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.core.env.Environment;
+import org.springframework.test.annotation.DirtiesContext;
+import org.springframework.test.context.TestPropertySource;
+import org.springframework.test.context.junit.jupiter.SpringJUnitConfig;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Integration tests for {@link TestPropertySource @TestPropertySource} support
+ * with custom resource encoding.
+ *
+ * @author Sam Brannen
+ * @since 6.1
+ */
+@SpringJUnitConfig
+class CustomEncodingTestPropertySourceTests {
+
+	@Nested
+	@TestPropertySource(locations = "test-ISO-8859-1.properties", encoding = "UTF-8" )
+	@DirtiesContext
+	class IncorrectEncodingTests {
+
+		@Test
+		void propertyIsAvailableInEnvironment(@Autowired Environment env) {
+			// The "é" characters in "Générales" get converted to U+FFFD : REPLACEMENT CHARACTER.
+			assertThat(env.getProperty("text")).isEqualTo("G\uFFFDn\uFFFDrales");
+		}
+	}
+
+	@Nested
+	@TestPropertySource(locations = "test-ISO-8859-1.properties", encoding = "ISO-8859-1" )
+	@DirtiesContext
+	class ExplicitEncodingTests {
+
+		@Test
+		void propertyIsAvailableInEnvironment(@Autowired Environment env) {
+			assertThat(env.getProperty("text")).isEqualTo("Générales");
+		}
+	}
+
+
+	@Configuration
+	static class Config {
+		/* no user beans required for these tests */
+	}
+
+}