Update validate to execute on every file individually

Per [this new requirement implemented in validate](NASA-PDS/validate#755), having many labels point to the same data file will cause validate to fail.

When we are testing with LDDs, we don't care about that, so let's validate each file individually.

src/pds/ldd_manager/validate.py

@@ -50,25 +50,26 @@ def exec_validate(executable, args, data_path, pds4_version, failure_expected=Fa
         if not files:
             continue
 
-        validate_args = args.copy()
-        validate_args.append('-t')
-        validate_args.extend(files)
-
-        cmd = ['bash', executable]
-        cmd.extend(validate_args)
-        with Popen(cmd, stdout=PIPE, stderr=STDOUT, bufsize=1, universal_newlines=True) as p:
-            with open(log_out, 'w') as f:
-                for line in p.stdout:
-                    print(line, end='') # process line here
-                    f.write(line)
-
-        success = False
-        if p.returncode != 0 and key == "fail":
-            success = True
-        elif p.returncode == 0 and key == "valid":
-            success = True
-        else:
-            raise CalledProcessError(p.returncode, p.args)
+        for file in files:
+            validate_args = args.copy()
+            validate_args.append('-t')
+            validate_args.extend(files)
+
+            cmd = ['bash', executable]
+            cmd.extend(validate_args)
+            with Popen(cmd, stdout=PIPE, stderr=STDOUT, bufsize=1, universal_newlines=True) as p:
+                with open(log_out, 'w') as f:
+                    for line in p.stdout:
+                        print(line, end='') # process line here
+                        f.write(line)
+
+            success = False
+            if p.returncode != 0 and key == "fail":
+                success = True
+            elif p.returncode == 0 and key == "valid":
+                success = True
+            else:
+                raise CalledProcessError(p.returncode, p.args)
 
     return success