1.0.46.23: fix an actual random bug in mkstemp test

1.0.46.23: fix an actual random bug in mkstemp test

 On some platforms (including Solaris, FWIW), the character "."  is one of
 the characters mkstemp can use when filling in an "XXXXXX" template.  When
 this (randomly) occurs during the running of this test, PATHNAME-NAME, of
 course breaks on the last such random "." character, and the test fails in
 a way which will not repeat until the next time such a character is
 randomly chosen.

 Since readdir actually returns a string, break the string on the first "."
 explicitly, instead of making a path from the string and calling
 pathname-name.

 This will need to be revisited if we add mkstemp support on win32 in the
 future.