[wp] use @PTEST_PLUGIN@ in tests

src/plugins/wp/tests/wp_acsl/checks.i

 /* run.config
-PLUGIN: wp,rtegen,scope,eva,report,inout
+PLUGIN: @PTEST_PLUGIN@,rtegen,scope,eva,report,inout
    OPT: -then -eva -then -report
-PLUGIN: wp,rtegen
+PLUGIN: @PTEST_PLUGIN@,rtegen
    OPT: -wp-prop=@check
    OPT: -wp-prop=-@check
 */

src/plugins/wp/tests/wp_plugin/removed.i

 /* run.config_qualif
-PLUGIN: wp,rtegen,eva,scope,inout
+PLUGIN: @PTEST_PLUGIN@,rtegen,eva,scope,inout
    OPT: -no-wp -eva -eva-msg-key=-summary -then -wp -then -no-eva -warn-unsigned-overflow -wp
  */
 /* run.config