test: Add a way to skip console checking until a string matches

Some tests produce a lot of output that does not need to be individually
checked by an assertion. Add a macro to handle this.

Series-to: u-boot
Series-links: 258653
Series-version: 2
Cover-letter:
sandbox: Minor fixes and improvements
This series collects a few patches that correct and improve sandbox and
sandbox_spl:

- Fix a bug in --rm-memory works
- Support an MMC backing file for MMC emulator, to allow filesystems to be
  used without needing the 'host' filesystem
- Allow skipping console output when checking verbose commands
- Suppress unwanted filesystem warnings when probing partitions
- Tidy up a few comments
END

Signed-off-by: Simon Glass <sjg@chromium.org>
Change-Id: I44fcf34ce986e97cb281a70e4fc04caed4c26045
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