support/testing/infra: add log_file_path() function

support/testing/infra: add log_file_path() function

Some tests will need to grep through the build log to verify that some
features are working are expected. In order to allow them to open the
build log, we provide a new function called log_file_path(), which
returns the path to the log file if available.

We also use this function in open_log_file().

Note that open_log_file() cannot be used directly to grep through the
log file at the end of a build: because it opens in "a+" mode, it
greps starting from the end of the file.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Herve Codina <herve.codina@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>