RFR: 7903869: Feature Tests - Fixing TT_SelectionCE test script

[Golla Yadav]

Base class "TT_SelectionCE" test script is executing, which is not required. So to avoid this adding below line in build.xml

<fileset dir="${tests.gui.srcpath}" excludes="*/TestTree/TT_SelectionCE.java" />

Also, added <unless="testfile"> condition to run the specific tests.

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Commit messages:
 - 7903869: Fixing TT_SelectionCE test script

Changes: https://git.openjdk.org/jtharness/pull/90/files
  Webrev: https://webrevs.openjdk.org/?repo=jtharness&pr=90&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903869
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jtharness/pull/90.diff
  Fetch: git fetch https://git.openjdk.org/jtharness.git pull/90/head:pull/90

PR: https://git.openjdk.org/jtharness/pull/90