Pass a test file to flex test.

test cases/frameworks/8 flex/meson.build

@@ -30,4 +30,5 @@ pfiles = pgen.process('parser.y')
 e = executable('pgen', 'prog.c',
 lfiles, pfiles)
 
-test('parsertest', e)
+test('parsertest', e,
+    args: [meson.current_source_dir() / 'testfile'])

test cases/frameworks/8 flex/prog.c

@@ -11,7 +11,7 @@ extern int yyparse();
 int main(int argc, char **argv) {
     int input;
     if(argc != 2) {
-        printf("%s <input file>");
+        printf("%s <input file>\n", argv[0]);
         return 1;
     }
     input = open(argv[1], O_RDONLY);

test cases/frameworks/8 flex/testfile

@@ -0,0 +1,2 @@
+true
+