RFR: 7903846: apidiff: test issues on Windows

[Jonathan Gibbons]

Please review a fix for some issues reported when running tests on Windows.

For `GetSerialVersionUITest`, the derivation of the working directory was broken in a couple of ways, which have both been fixed:
1. the computation of the path name assumed all members were fields; whereas the test cases have evolved to include other complex members
2. the work/scratch directory was being emptied between test cases, so that only the last one remained. there is a more appropriate method to use instead, so that all the working files are preserved

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Commit messages:
 - CODETOOLS-7903846: apidiff: test issues on Windows

Changes: https://git.openjdk.org/apidiff/pull/16/files
  Webrev: https://webrevs.openjdk.org/?repo=apidiff&pr=16&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903846
  Stats: 12 lines in 2 files changed: 2 ins; 0 del; 10 mod
  Patch: https://git.openjdk.org/apidiff/pull/16.diff
  Fetch: git fetch https://git.openjdk.org/apidiff.git pull/16/head:pull/16

PR: https://git.openjdk.org/apidiff/pull/16