[foreign-memaccess+abi] RFR: Fix test with strcat

[Per Minborg]

Fix test with strcat

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Commit messages:
 - Fix allocation size
 - Fix test with strcat

Changes: https://git.openjdk.org/panama-foreign/pull/860/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=860&range=00
  Stats: 6 lines in 1 file changed: 5 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/860.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign.git pull/860/head:pull/860

PR: https://git.openjdk.org/panama-foreign/pull/860