[deckhouse-cli] test: cover clean stdout during command startup

[fuldaxxx]

Description

Adds regression coverage for the invariant that d8 writes nothing to stdout while building its command tree. Two layers, no production code changes:

• cmd/d8/root_test.go — in-process unit test: swaps os.Stdout for a pipe, points DECKHOUSE_CLI_PATH at a path that can be neither created nor read (its parent is a regular file, so ENOTDIR holds for root too), and requires the capture to stay empty after NewRootCommand(). A second test asserts the capture actually sees a log line written the way the CLI logger writes one, so the first cannot pass vacuously.
• testing/e2e/auth/exec_credential_e2e_test.go — same invariant on the built binary: runs d8 --version as a subprocess under the same unreadable plugins path and requires stdout to hold exactly one line, the version output. This is the only layer that covers file descriptor 1 as a whole — build tags, linked-in libraries, anything writing outside this repository's logger. The binary is taken from D8_BIN, then bin/d8, then build/<goos>-<goarch>/bin/d8; with none present the test skips, since a clean checkout has no artifact to check and the unit test is what gates CI.

Both tests pin LOG_LEVEL explicitly so an ambient LOG_LEVEL=debug cannot turn them red for an unrelated, still-open defect (see below).