RFR: 8299553: Make ScaledEtchedBorderTest.java comprehensive

[Alexey Ivanov]

The `ScaledEtchedBorderTest.java` test ([JDK-8279614](https://bugs.openjdk.org/browse/JDK-8279614)) verifies that the thickness of lines of `EtchedBorder` is consistent. The component inside had the same background color as the frame. Yet if there's another component the background of which is different, that background could be seen.

The updated test covers the scenario reported in [JDK-8294921](https://bugs.openjdk.org/browse/JDK-8294921).

The bug itself is fixed by refactoring [JDK-8294680](https://bugs.openjdk.org/browse/JDK-8294680) the border rendering code which uses a more precise coordinate calculation, see #11571. Thus, I contribute the test under a separate bugid.

I also modified `ScaledLineBorderTest.java` so that the location of the child panels is saved when they're created as done in `ScaledTextFieldBorderTest.java` in #11498. I removed the unused `Component` import after this modification from both tests.

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Commit messages:
 - Drop the unused import: Component
 - Save panel locations when creating them
 - 8299553: Make ScaledEtchedBorderTest.java comprehensive

Changes: https://git.openjdk.org/jdk/pull/11836/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11836&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8299553
  Stats: 280 lines in 3 files changed: 141 ins; 50 del; 89 mod
  Patch: https://git.openjdk.org/jdk/pull/11836.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11836/head:pull/11836

PR: https://git.openjdk.org/jdk/pull/11836