Fix MessageBufferInputTest flake: mkdirs() before createTempFile

saveToTmpFile called File.createTempFile("testbuf", ".dat", new
File("target")) before tmp.getParentFile.mkdirs() — createTempFile
requires the parent directory to already exist, so under occasional
filesystem timing in CI sandboxes this threw "No such file or directory"
before the directory got created. Surfaced while validating the sbt 2
migration's CI matrix; unrelated to sbt 2 itself, but was intermittently
failing the JDK8 test lane via fail-fast cancellation of the rest of the
matrix.

Author: xerial

msgpack-core/src/test/scala/org/msgpack/core/buffer/MessageBufferInputTest.scala

@@ -58,8 +58,9 @@ class MessageBufferInputTest extends AirSpec:
     def toByteBuffer = ByteBuffer.wrap(b)
 
     def saveToTmpFile: File =
-      val tmp = File.createTempFile("testbuf", ".dat", new File("target"))
-      tmp.getParentFile.mkdirs()
+      val dir = new File("target")
+      dir.mkdirs()
+      val tmp = File.createTempFile("testbuf", ".dat", dir)
       tmp.deleteOnExit()
       withResource(new FileOutputStream(tmp)) { out =>
         out.write(b)