tests: add test for malformed versions generating a diagnostic with --validate

tests/parser.sh

@@ -33,7 +33,8 @@ tests_init \
 	msvc_fragment_render_cflags \
 	tuple_dequote \
 	version_with_whitespace \
-	version_with_whitespace_2
+	version_with_whitespace_2 \
+	version_with_whitespace_diagnostic
 
 comments_body()
 {
@@ -299,3 +300,10 @@ version_with_whitespace_2_body()
 		-o inline:'malformed-version = 3.922\n' \
 		pkgconf --with-path="${selfdir}/lib1" --print-provides malformed-version
 }
+
+version_with_whitespace_diagnostic_body()
+{
+	atf_check \
+		-o match:warning \
+		pkgconf --with-path="${selfdir}/lib1" --validate malformed-version
+}