btrfs: annotate btrfs_is_testing() as unlikely and make it return bool

We can annotate btrfs_is_testing() as unlikely since that's the most
expected scenario and it's desirable for the compiler to optimize for
the case we are not running the self tests. So add the annotation to
btrfs_is_testing() and while at it also make it return bool instead of
int.

Also make two of the existing callers use btrfs_is_testing() directly
instead of storing its result in a local variable.

On x86_64 with Debian's gcc 14.2.0-19 this resulted in a very tiny object
code reduction.

Before this change:

  $ size fs/btrfs/btrfs.ko
     text	   data	    bss	    dec	    hex	filename
  1913263	 161567	  15592	2090422	 1fe5b6	fs/btrfs/btrfs.ko

After this change:

  $ size fs/btrfs/btrfs.ko
     text	   data	    bss	    dec	    hex	filename
  1913257	 161567	  15592	2090416	 1fe5b0	fs/btrfs/btrfs.ko

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>