btrfs: tests: print file:line for error messages

For better diagnostics print the file name and line to locate the
errors. Sample output:

[    9.052924] BTRFS: selftest: fs/btrfs/tests/extent-io-tests.c:283 offset bits do not match
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/tests/btrfs-tests.h

@@ -10,7 +10,8 @@
 int btrfs_run_sanity_tests(void);
 
 #define test_msg(fmt, ...) pr_info("BTRFS: selftest: " fmt "\n", ##__VA_ARGS__)
-#define test_err(fmt, ...) pr_err("BTRFS: selftest: " fmt "\n", ##__VA_ARGS__)
+#define test_err(fmt, ...) pr_err("BTRFS: selftest: %s:%d " fmt "\n",	\
+		__FILE__, __LINE__, ##__VA_ARGS__)
 
 struct btrfs_root;
 struct btrfs_trans_handle;