[foreign-abi] RFR: Add test for vprintf

[Maurizio Cimadamore]

For completeness, I added a test for `vprintf` in our `StdLibTest`. The logic is the same as for the `printf` test, but
instead of passing arguments separately to a specialized handle, we instead construct the correct valist object which
is then passed to the shared `vprintf` handle.

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Commit messages:
 - Add test for vprintf

Changes: https://git.openjdk.java.net/panama-foreign/pull/205/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/205/webrev.00
  Stats: 48 lines in 1 file changed: 42 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/205.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/205/head:pull/205

PR: https://git.openjdk.java.net/panama-foreign/pull/205