Trac #29442: Fix failing doctest in src/sage/interfaces/tachyon.py

{{{
sage -t src/sage/interfaces/tachyon.py
**********************************************************************
File "src/sage/interfaces/tachyon.py", line 175, in
sage.interfaces.tachyon.TachyonRT.usage
Failed example:
    t.usage(use_pager=False)
Expected:
    Tachyon Parallel/Multiprocessor Ray Tracer   Version...
Got:
    Usage: tachyon modelfile [options]
    <BLANKLINE>
    Valid options and accepted formats are as follows (** denotes
default behaviour).
    <BLANKLINE>
    Model file formats supported:
...
}}}

This seems to have been introduced by using the systems tachyon. It
appears that in some cases (depending on the version probably) tachyon
does not print out the first lines
{{{
Tachyon Parallel/Multiprocessor Ray Tracer   Version 0.98.9
Copyright 1994-2010,    John E. Stone <[email protected]>
------------------------------------------------------------
}}}

We fix this by testing for the following output:
{{{
...
Usage:
  tachyon modelfile [options]...
<BLANKLINE>
Model file formats supported:
  filename.dat ...
}}}

URL: https://trac.sagemath.org/29442
Reported by: gh-kliem
Ticket author(s): Jonathan Kliem
Reviewer(s): Frédéric Chapoton, Matthias Koeppe

Author: Release Manager

src/sage/interfaces/tachyon.py

@@ -74,7 +74,7 @@ class TachyonRT(SageObject):
 
     EXAMPLES:
 
-    
+
     .. automethod:: __call__
     """
     def _repr_(self):
@@ -173,7 +173,11 @@ def usage(self, use_pager=True):
             sage: from sage.interfaces.tachyon import TachyonRT
             sage: t = TachyonRT()
             sage: t.usage(use_pager=False)
-            Tachyon Parallel/Multiprocessor Ray Tracer   Version...
+            ...
+              tachyon modelfile [options]...
+            <BLANKLINE>
+            Model file formats supported:
+              filename.dat ...
         """
         with os.popen('tachyon') as f:
             r = f.read()