bug#36940: tests slowness and failure after recent Tramp changes

[Paul Eggert]

Michael Albinus wrote:
> > Second, the tramp-test19-directory-files-and-attributes test now fails.
> Hmm, prior pushing the patch yesterday, all tests have passed. Today, I
> can reproduce the problem. Maybe some of the time conversion changes,
> arrived over night, have caused this failure? Will check.

Thanks for looking into it. The failure started with commit 
2019-08-05T11:09:26Z!michael.albinus@gmx.de 
(6c1d0d53b34d9350d55ebbd83ea56aa751a55f1b), which predates those time-conversion 
changes.

> If you want to run the
> default tests, which are active while "make check", you must use the
> default selector. See test/README.

Thanks, I didn't know that. But that's confusing, as it's natural to assume that 
if FOO is an individual check, then 'make FOO' will run it the same way that 
'make check' would. Would it make sense to change this test to be more natural?