TST: avoid shadowing errors in test sources.

When some unit test module fails to import, use the discovered
test suite as is without any test selection.  Raise exception
relevant to the module error rather then a later one from
test selection failure.

Author: pavoljuhas

src/diffpy/srfit/tests/__init__.py

@@ -33,7 +33,8 @@ def testsuite(pattern=''):
     ----------
     pattern : str, optional
         Regular expression pattern for selecting test cases.
-        Select all tests when empty.
+        Select all tests when empty.  Ignore the pattern when
+        any of unit test modules fails to import.
 
     Returns
     -------
@@ -54,7 +55,11 @@ def testsuite(pattern=''):
     # always filter the suite by pattern to test-cover the selection code.
     suite = unittest.TestSuite()
     rx = re.compile(pattern)
-    tcases = chain.from_iterable(chain.from_iterable(suite_all))
+    tsuites = list(chain.from_iterable(suite_all))
+    tsok = all(isinstance(ts, unittest.TestSuite) for ts in tsuites)
+    if not tsok:    # pragma: no cover
+        return suite_all
+    tcases = chain.from_iterable(tsuites)
     for tc in tcases:
         tcwords = tc.id().rsplit('.', 2)
         shortname = '.'.join(tcwords[-2:])