DiffToolTest: don't assume temp files use java.io.tmpdir

testToolCached compared the temp file path that "git difftool --staged"
creates against System.getProperty("java.io.tmpdir"). Inside the
Surefire fork those two name different directories. In order:

- the fork starts with -Djava.io.tmpdir=${project.build.directory}
  from the pgm.test pom, so java.io.tmpdir is <module>/target at startup
- File.createTempFile captures that startup value and writes under
  <module>/target
- Maven is run with -Djava.io.tmpdir=<dir> (as CI does), and Surefire
  applies it with System.setProperty after the fork has started
- System.getProperty therefore now returns <dir>, not <module>/target
- the test expects <dir>/<name>_REMOTE_* but the file is under
  <module>/target, so the match fails

This surfaced with the Java 17 -> 25 bump: on Java 17
File.createTempFile read java.io.tmpdir live and so agreed with
System.getProperty; since Java 21 it uses the value captured at JVM
startup, so the two diverge once Surefire changes the property after
the fork has started.

Neutralize this JDK behavioral change by making no assumption about
where the temporary directory comes from: match only the file name
pattern, not its directory, so the test harness is JDK-agnostic.

Change-Id: I9c48f8c449301230f5a3a71b4ca98c1be911c8d6