selftests/bpf: Silence ima_setup.sh when not running in verbose mode.

Currently, ima_setup.sh spews outputs from commands like mkfs and dd
on the terminal without taking into account the verbosity level of
the test framework. Update test_progs to set the environment variable
SELFTESTS_VERBOSE=1 when a verbose output is requested. This
environment variable is then used by ima_setup.sh (and can be used by
other similar scripts) to obey the verbosity level of the test harness
without needing to re-implement command line options for verbosity.

In "silent" mode, the script saves the output to a temporary file, the
contents of which are echoed back to stderr when the script encounters
an error.

Fixes: 34b82d3a ("bpf: Add a selftest for bpf_ima_inode_hash")
Reported-by: Andrii Nakryiko <andrii@kernel.org>
Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: KP Singh <kpsingh@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20201211010711.3716917-1-kpsingh@kernel.org