RFR: 7903846: apidiff: test issues on Windows [v2]

[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

Jonathan Gibbons has updated the pull request incrementally with one additional commit since the last revision:

  review feedback: proactively remove edit to JUnitTests.gmk to avoid a merge conflict with an equivalent change in another PR.

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Changes:
  - all: https://git.openjdk.org/apidiff/pull/16/files
  - new: https://git.openjdk.org/apidiff/pull/16/files/616a1d77..c1463766

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=apidiff&pr=16&range=01
 - incr: https://webrevs.openjdk.org/?repo=apidiff&pr=16&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 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