tests: exclude bundled pywatchman from test-check-pyflakes

The code under pywatchman used to be excluded from test-check-code and
test-check-pyflakes through the magic string "no-check-code". Now that
test-check-code excludes the pywatchman directory entirely, the directory
should also be excluded from test-check-pyflakes.

tests/test-check-pyflakes.t

@@ -6,7 +6,7 b''
 run pyflakes on all tracked files ending in .py or without a file ending
 (skipping binary file random-seed)
 
-  $ hg locate 'set:**.py or grep("^#!.*python")' \
+  $ hg locate 'set:**.py or grep("^#!.*python")' -X hgext/fsmonitor/pywatchman \
   > -X mercurial/pycompat.py \
   > 2>/dev/null \
   > | xargs pyflakes 2>/dev/null | "$TESTDIR/filterpyflakes.py"