RFR: 8256154: Some TestNG tests require default constructors

[Conor Cleary]

In TestNG 7, it is a requirement that TestNG is able to create a Test object using a default constructor. 

This simple fix addresses two such classes so that this requirement is satisfied by inserting default construtors. Example: `public GetPackages() { ... }`

test/jdk/java/lang/Package/GetPackages.java
test/jdk/java/lang/StackWalker/Basic.java

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Commit messages:
 - 8256154: Some TestNG tests require default constructors

Changes: https://git.openjdk.java.net/jdk/pull/1317/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1317&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256154
  Stats: 13 lines in 2 files changed: 11 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1317.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1317/head:pull/1317

PR: https://git.openjdk.java.net/jdk/pull/1317