RFR: 7902986: Sigtest should better handle permitted classes

[Victor Rudometov]

Fixed the issue when during runtime Sigtest failed to load permitted classes information for sealed classes.

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Commit messages:
 - CODETOOLS-7902986 Sigtest should better handle permitted classes

Changes: https://git.openjdk.java.net/sigtest/pull/4/files
 Webrev: https://webrevs.openjdk.java.net/?repo=sigtest&pr=4&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7902986
  Stats: 21 lines in 1 file changed: 2 ins; 5 del; 14 mod
  Patch: https://git.openjdk.java.net/sigtest/pull/4.diff
  Fetch: git fetch https://git.openjdk.java.net/sigtest pull/4/head:pull/4

PR: https://git.openjdk.java.net/sigtest/pull/4