Add support for property file encoding in `@TestPropertySource`

[sbrannen]

Overview

In Spring Framework 4.3, we introduced support for property file encoding in @PropertySource. This issue aims to introduce the same support in @TestPropertySource.

Related Issues

• Add support for UTF-8 and other charset/encoding to @PropertySource [SPR-13874] #18447
• Support custom properties file formats in @TestPropertySource #30981