Add Parsec Subsection Warning to test

Expects Invalid subsection warning by the Parsec ProjectConfig parser.
Currently the warning is issued twice:
First warning: "Warning: <ROOT>/else.project, else.project: Unrecognized
section '_' on line 3" is issued by the legacy parser,
Second warning: "Warning: dir-else/else.config:3:5: Invalid subsection
"_"" by Parsec Parser.
When we remove executing the legacy parser, we can remove the duplicate warning.

cabal-testsuite/PackageTests/ProjectImport/ParseErrorProvenance/cabal.out

@@ -1,5 +1,6 @@
 # cabal v2-build
 Warning: <ROOT>/else.project, else.project: Unrecognized section '_' on line 3
+Warning: dir-else/else.config:3:5: Invalid subsection "_"
 # Multiline string marking:
 # ^When using configuration from:$
 # ^  - else.project$