chore: replace testing.Testing with flag lookup (#26552)

In our codebase we have an existing convention of using
`flag.Lookup("test.v")` instead of `testing.Testing()`. This avoids
pulling in the entire `testing` package. Another consequence: some of
our custom linters trigger upon import of the `testing` package which
can lead to unexpected linter errors.

coderd/database/dbauthz/dbauthz.go

@@ -5,10 +5,10 @@ import (
 	"database/sql"
 	"encoding/json"
 	"errors"
+	"flag"
 	"slices"
 	"strings"
 	"sync/atomic"
-	"testing"
 	"time"
 
 	"github.com/google/uuid"
@@ -2611,7 +2611,7 @@ func (q *querier) DeleteWorkspaceSubAgentByID(ctx context.Context, id uuid.UUID)
 }
 
 func (q *querier) DisableForeignKeysAndTriggers(ctx context.Context) error {
-	if !testing.Testing() {
+	if flag.Lookup("test.v") == nil {
 		return xerrors.Errorf("DisableForeignKeysAndTriggers is only allowed in tests")
 	}
 	return q.db.DisableForeignKeysAndTriggers(ctx)