add newline in error message between code path and actual failed line in test file

[msbutler]

Currently, data driven tests return an error message that looks like:

    --- FAIL: TestBackupRestoreDataDriven/column-families (7.88s)
        backup_test.go:283: 
            /private/var/tmp/_bazel_michaelbutler/13ba282fa2b19539d0c969c1113bb37c/sandbox/darwin-sandbox/1/execroot/cockroach/bazel-out/darwin-fastbuild/bin/pkg/ccl/backupccl/backupccl_test_/backupccl_test.runfiles/cockroach/pkg/ccl/backupccl/testdata/backup-restore/column-families:52: SELECT max(length(start_key)) FROM [SHOW RANGES FROM TABLE r1.cfs];
            expected:
            2
            
            found:
            <nil>
    backup_test.go:379: -- test log scope end --
FAIL
I220324 21:54:59.442315 1 (gostd) testmain.go:432  [-] 1  Test //pkg/ccl/backupccl:backupccl_test exited with error code 1


ERROR: <nil>

202 runs completed, 1 failures, over 2m37s
context canceled
FAIL

A more user friendly error would add a newline between the code path and the actual line that failed, i.e. (you can see the SELECT statement in question below):

    --- FAIL: TestBackupRestoreDataDriven/column-families (7.88s)
        backup_test.go:283: 
            /private/var/tmp/_bazel_michaelbutler/13ba282fa2b19539d0c969c1113bb37c/sandbox/darwin-sandbox/1/execroot/cockroach/bazel-out/darwin-fastbuild/bin/pkg/ccl/backupccl/backupccl_test_/backupccl_test.runfiles/cockroach/pkg/ccl/backupccl/testdata/backup-restore/column-families:52: 
           SELECT max(length(start_key)) FROM [SHOW RANGES FROM TABLE r1.cfs];
            expected:
            2
            
            found:
            <nil>
    backup_test.go:379: -- test log scope end --
FAIL
I220324 21:54:59.442315 1 (gostd) testmain.go:432  [-] 1  Test //pkg/ccl/backupccl:backupccl_test exited with error code 1


ERROR: <nil>

202 runs completed, 1 failures, over 2m37s
context canceled
FAIL

The lack of newline lead to some confusion diagnosing a data driven test failure: here and here.

[msbutler]

i beleive the fix is adding another \n between the first and second arg in the string on https://github.com/cockroachdb/datadriven/blob/master/datadriven.go#L369

happy to open a PR with the fix if folks think this would be a good idea.