Repair modparser tests

### What does this PR do?
Repair the `modparser` tests that were inadvertently broken, and remove
unused/outdated `testdata/*/go.sum` files.

### Motivation
Offload #45714 which aims at migrating `modparser` as first Go module to
`bazel` (better have them individually cherry-pickable/revertable).

The tests were broken since #30061 when `require` blocks were removed
from `testdata/*/go.mod` files during a dependencies upgrade.
As the tests are not yet (or no longer?) running in CI, the breakage
went unnoticed.

### Describe how you validated your changes
- test failure: `TestFilterMatch/With_matches` expected to find
  `["github.com/DataDog/datadog-agent/pkg/test"]` but got empty array.
  All 7/7 test cases now pass.
- golangci-lint issues: 2 `perfsprint` violations (`fmt.Errorf` without
  format specifiers should use `errors.New`).

### Additional Notes
#45714 or a follow-up PR will make sure the tests run in CI.

Author: rdesgroppes

internal/tools/modparser/main_test.go

@@ -6,7 +6,7 @@
 package main
 
 import (
-	"fmt"
+	"errors"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -36,12 +36,12 @@ func TestParseMod(t *testing.T) {
 		{
 			name:        "Missing module",
 			modPath:     "./testdata/nonexistant/",
-			expectedErr: fmt.Errorf("could not read go.mod file in ./testdata/nonexistant/"),
+			expectedErr: errors.New("could not read go.mod file in ./testdata/nonexistant/"),
 		},
 		{
 			name:        "Badly formatted module",
 			modPath:     "./testdata/badformat/",
-			expectedErr: fmt.Errorf("could not parse go.mod file in ./testdata/badformat/"),
+			expectedErr: errors.New("could not parse go.mod file in ./testdata/badformat/"),
 		},
 	}
 

internal/tools/modparser/testdata/match/go.mod

@@ -1,3 +1,7 @@
 module github.com/DataDog/datadog-agent/internal/tools/modparser/testdata/match
 
-go 1.18
+go 1.25.0
+
+require (
+    github.com/DataDog/datadog-agent/pkg/test v0.65.3
+)

internal/tools/modparser/testdata/match/go.sum

@@ -1,3 +1,8 @@
 module github.com/DataDog/datadog-agent/internal/tools/modparser/testdata/nomatch
 
-go 1.18
+go 1.25.0
+
+require (
+    github.com/DataDog/datadog-not-agent/pkg/test v0.65.3
+    not/github.com/DataDog/datadog-agent/pkg/test v0.65.3
+)