test/tap/tests: wire mysql-zstd_compression_level_libmysql-t into tests target

Commit 174c5bd added the pattern rule for the _libmysql variant of the
zstd_compression_level test but did not add it to the 'tests:' target
list, so 'make build_tap_tests' silently skipped building it. Add the
missing entry so the libmysql variant is built alongside its siblings.

Author: renecannao

test/tap/tests/Makefile

@@ -132,6 +132,7 @@ tests: tests-cpp \
 	prepare_statement_err3024_libmysql-t \
 	prepare_statement_err3024_async-t \
 	fast_forward_grace_close_libmysql-t \
+	mysql-zstd_compression_level_libmysql-t \
 	fast_forward_switch_replication_deprecate_eof_libmysql-t \
 	reg_test_mariadb_stmt_store_result_libmysql-t \
 	reg_test_mariadb_stmt_store_result_async-t