RFR: 8274265: Suspicious string concatenation in test code

[Jesper Steen Møller]

This patch fixes the mishap, and lets JDK compile under XCode 13 / LLVM 13.

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Commit messages:
 - 8274265: Suspicious string concatenation in test code

Changes: https://git.openjdk.java.net/jdk/pull/5675/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5675&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274265
  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5675.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5675/head:pull/5675

PR: https://git.openjdk.java.net/jdk/pull/5675