Add deprecation test for old configuration entry

Author: bossmc

tests/ui-toml/conf_deprecated_key/clippy.toml

@@ -1,5 +1,6 @@
-# that one is a warning
+# Expect errors from these deprecated configs
 cyclomatic-complexity-threshold = 2
+blacklisted-names = [ "..", "wibble" ]
 
 # that one is white-listed
 [third-party]

tests/ui-toml/conf_deprecated_key/conf_deprecated_key.stderr

@@ -1,5 +1,7 @@
 warning: error reading Clippy's configuration file `$DIR/clippy.toml`: deprecated field `cyclomatic-complexity-threshold`. Please use `cognitive-complexity-threshold` instead
 
+warning: error reading Clippy's configuration file `$DIR/clippy.toml`: deprecated field `blacklisted-names`. Please use `disallowed-names` instead
+
 error: the function has a cognitive complexity of (3/2)
   --> $DIR/conf_deprecated_key.rs:4:4
    |
@@ -9,5 +11,5 @@ LL | fn cognitive_complexity() {
    = note: `-D clippy::cognitive-complexity` implied by `-D warnings`
    = help: you could split it up into multiple smaller functions
 
-error: aborting due to previous error; 1 warning emitted
+error: aborting due to previous error; 2 warnings emitted