btrfs-progs: tests: add support for command instrumentation

btrfs-progs: tests: add support for command instrumentation

Add a way to wrap commands executed by the tests. This means the
common wrappers: run_check, run_check_stdout and run_mayfail , with the
exception of the use root_helper.

The contents of the shell variable INSTRUMENT are prepended to the
command, without quotes. Use with care.

Example: this has been tested with valgrind, the output goes to the
RESULTS file.

 $ INSTRUMENT=valgrind make test-misc

Any use of root_helper/SUDO_HELPER will skip the instrumentation.

Signed-off-by: David Sterba <dsterba@suse.com>