test: Show warning when a `-filter` flag has an invalid value (#38603)

* test: Show warning when a test file filter is not valid

* Add change file

.changes/v1.16/BUG FIXES-20260515-201307.yaml

@@ -0,0 +1,5 @@
+kind: BUG FIXES
+body: 'test: Terraform will now raise a warning when a file referenced via `-filter` flag does not exist.'
+time: 2026-05-15T20:13:07.430709+01:00
+custom:
+    Issue: "38603"

internal/backend/local/test.go

@@ -180,7 +180,7 @@ func (runner *TestSuiteRunner) collectTests() (*moduletest.Suite, tfdiags.Diagno
 						// If the filter is invalid, we'll simply skip this
 						// entry and print a warning. But we could still execute
 						// any other tests within the filter.
-						diags.Append(tfdiags.Sourceless(
+						diags = diags.Append(tfdiags.Sourceless(
 							tfdiags.Warning,
 							"Unknown test file",
 							fmt.Sprintf("The specified test file, %s, could not be found.", name)))

internal/command/test_test.go

@@ -179,6 +179,15 @@ func TestTest_Runs(t *testing.T) {
 			expectedOut: []string{"1 passed, 0 failed"},
 			code:        0,
 		},
+		"multiple_files_with_invalid_filter": {
+			override: "multiple_files",
+			args:     []string{"-filter=nonexistent.tftest.hcl"},
+			expectedOut: []string{
+				"Success! 0 passed, 0 failed.",
+				"Warning: Unknown test file",
+			},
+			code: 0,
+		},
 		"no_state": {
 			expectedOut: []string{"0 passed, 1 failed"},
 			expectedErr: []string{"No value for required variable"},