[ClassLoader] Fix class names in ApcUniversalClassLoader tests.

[ClassLoader] These tests were failing due to a leading slash in the class name. Support for the leading slash in findFile was removed a while ago, but the Apc tests were not updated. See #10576 for more details.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #10576
| License       | MIT
| Doc PR        | N/A