Move platform independent tests to a shared runfile

Move platform independent tests to a shared runfile

Tests that aren't limited to running on Linux can be moved to a common
runfile to be shared with other platforms.

The test runner and wrapper script are enhanced to allow specifying
multiple runfiles as a comma-separated list. The default runfiles are
now "common.run,PLATFORM.run" where PLATFORM is determined at run time.

Sections in runfiles that share a path with another runfile can append
a colon separator and an identifier to the path in the section
name, ie `[tests/functional/atime:Linux]`, to avoid overriding the tests
specified by other runfiles.

Reviewed-by: Jorgen Lundman <lundman@lundman.net>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Ryan Moeller <ryan@ixsystems.com>
Closes #9391