pgrep: Fix off by one error in line check

There is now a warning if your command is longer than 15 characters
and therefore can never match. Except it was checking for more than
16 characters.

Adjusted this and added a test case.

References:
 !25
 commit 8e8835b2ee

testsuite/pgrep.test/pgrep.exp

@@ -118,6 +118,9 @@ set test "pgrep does not match substring with exact"
 spawn $pgrep -x $testproc_trim
 expect_blank $test
 
+set test "pgrep with long match gives warning"
+spawn $pgrep gnome-session-bi
+expect_pass "$test" "pattern that contains program name longer than 15 characters will result in zero matches"
 
 # Cleanup
 kill_testproc