test-serve: ensure KILLQUIETLY is only used once

49463314c24f introduced a feature to the hgserve() function that
suppresses errors from kill(1). It wrongly assumed that setting an
environment variable when calling a function would make it local to
the function. It ended up suppressing kill errors for every call
thereafter.

This patch sets KILLQUIETLY=N after use.

tests/test-serve

@@ -41,7 +41,7 b' echo % With -v and -p HGPORT2'
 hgserve -p "$HGPORT2"
 
 echo '% With -v and -p http (should fail)'
-KILLQUIETLY=Y hgserve -p http
+KILLQUIETLY=Y hgserve -p http; KILLQUIETLY=N
 
 echo % With --prefix foo
 hgserve --prefix foo