[foreign-jextract] RFR: Do not try to resize constant string segments in TestClassGeneration.java

[Jorn Vernee]

Hi,

This fixes a problem with the TestClassGeneration test, which tries to resize a MemrorySegment of a string constant,
but this is not needed, since the constants will already have a segment of the right size. This is causing a failure
since Foreign::withSize requires an unchecked address as input.

This patch replaces the withSize call to an assertion on the size.

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Commit messages:
 - Do not try to resize constant string segments in TestClassGeneration.java

Changes: https://git.openjdk.java.net/panama-foreign/pull/120/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/120/webrev.00
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/120.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/120/head:pull/120

PR: https://git.openjdk.java.net/panama-foreign/pull/120