tests: don't wipe python environment

* tests/run.in: keep original PYTHONPATH contents
* NEWS: mention the bug

NEWS

@@ -13,6 +13,9 @@ New in spot 2.10.4.dev  (net yet released)
   - Do not use the seq command in test cases, it is not available
     everywhere.
 
+  - Do not erase the previous contents of the PYTHONPATH environment
+    variable when running tests, prepend to it instead.
+
 New in spot 2.10.4  (2022-02-01)
 
   Bug fixed:

tests/run.in

@@ -104,18 +104,21 @@ export srcdir
 
 case $1 in
   *.ipynb)
-    PYTHONPATH=$pypath DYLD_LIBRARY_PATH=$modpath:$DYLD_LIBRARY_PATH \
+    PYTHONPATH=$pypath:$PYTHONPATH \
+    DYLD_LIBRARY_PATH=$modpath:$DYLD_LIBRARY_PATH \
     PYTHONIOENCODING=utf-8:surrogateescape \
     exec $PREFIXCMD @PYTHON@ @abs_srcdir@/python/ipnbdoctest.py "$@";;
   *.py)
-    PYTHONPATH=$pypath DYLD_LIBRARY_PATH=$modpath:$DYLD_LIBRARY_PATH \
+    PYTHONPATH=$pypath:$PYTHONPATH \
+    DYLD_LIBRARY_PATH=$modpath:$DYLD_LIBRARY_PATH \
     exec $PREFIXCMD @PYTHON@ "$@";;
   *.test)
     exec sh -x "$@";;
   *.pl)
     exec $PERL "$@";;
   *python*|*jupyter*|*pypy*)
-    PYTHONPATH=$pypath DYLD_LIBRARY_PATH=$modpath:$DYLD_LIBRARY_PATH \
+    PYTHONPATH=$pypath:$PYTHONPATH \
+    DYLD_LIBRARY_PATH=$modpath:$DYLD_LIBRARY_PATH \
     exec $PREFIXCMD "$@";;
   *)
     echo "Unknown extension" >&2