RFR: 8353321: [macos] ErrorTest.testAppContentWarning test case requires signing environment

[Alexey Semenyuk]

Move `ErrorTest.testAppContentWarning()` test case to `MacSignTest.testAppContentWarning()` and rework it to fail because of faulty app content and not invalid signing identity.

MacSignTest is supposed to be a placeholder for tests that require a signing environment but don't fit in any existing signing tests.

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Commit messages:
 - Move ErrorTest.testAppContentWarning() to signing tests

Changes: https://git.openjdk.org/jdk/pull/24338/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24338&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8353321
  Stats: 145 lines in 4 files changed: 107 ins; 29 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/24338.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24338/head:pull/24338

PR: https://git.openjdk.org/jdk/pull/24338