RFR: 8286620: Create regression test for verifying setMargin() of JRadioButton [v3]

[Tejesh R]

> Added test for checking setMargin() of JRadioButton.

Tejesh R has updated the pull request incrementally with two additional commits since the last revision:

 - Updated based on review comments
 - Added headful key

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8721/files
  - new: https://git.openjdk.java.net/jdk/pull/8721/files/2635e935..3001e8a8

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8721&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8721&range=01-02

  Stats: 7 lines in 1 file changed: 1 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8721.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8721/head:pull/8721

PR: https://git.openjdk.java.net/jdk/pull/8721