Make "sage -t -all" more robust, in particular when SAGE_SRC is SAGE_LIB

[mkoeppe]

Observed on conda:

$ sage -t --all
too many failed tests, not using stored timings
Running doctests with ID 2021-02-06-16-05-28-bc4dacc2.
Using --optional=dochtml,pip,sage
Doctesting entire Sage library.
Traceback (most recent call last):
  File "/Users/mkoeppe/miniconda3/envs/sage/bin/sage-runtests", line 182, in <module>
    err = DC.run()
  File "/Users/mkoeppe/miniconda3/envs/sage/lib/python3.9/site-packages/sage/doctest/control.py", line 1234, in run
    self.expand_files_into_sources()
  File "/Users/mkoeppe/miniconda3/envs/sage/lib/python3.9/site-packages/sage/doctest/control.py", line 817, in expand_files_into_sources
    self.sources = [FileDocTestSource(path, self.options) for path in expand()]
  File "/Users/mkoeppe/miniconda3/envs/sage/lib/python3.9/site-packages/sage/doctest/control.py", line 817, in <listcomp>
    self.sources = [FileDocTestSource(path, self.options) for path in expand()]
  File "/Users/mkoeppe/miniconda3/envs/sage/lib/python3.9/site-packages/sage/doctest/sources.py", line 528, in __init__
    raise ValueError("unknown file extension %r"%ext)
ValueError: unknown file extension ''