test: replaced use of python with $NOTMUCH_PYTHON (twice)

$NOTMUCH_PYTHON is sourced from sh.config, configured by
./configure and stated to be used as:

"Name of python command to use in configure and the test suite."

test/T260-thread-order.sh

@@ -4,7 +4,7 @@ test_description="threading when messages received out of order"
 
 # Generate all single-root four message thread structures.  We'll use
 # this for multiple tests below.
-THREADS=$(python ${TEST_DIRECTORY}/gen-threads.py 4)
+THREADS=$($NOTMUCH_PYTHON ${TEST_DIRECTORY}/gen-threads.py 4)
 nthreads=$(wc -l <<< "$THREADS")
 
 test_begin_subtest "Messages with one parent get linked in all delivery orders"

test/test-lib.sh

@@ -667,7 +667,7 @@ test_expect_equal_json () {
 
 # Sort the top-level list of JSON data from stdin.
 test_sort_json () {
-    PYTHONIOENCODING=utf-8 python -c \
+    PYTHONIOENCODING=utf-8 $NOTMUCH_PYTHON -c \
         "import sys, json; json.dump(sorted(json.load(sys.stdin)),sys.stdout)"
 }