test: include cmdline in captured output (#5977)

aiming for faster to understand a bunch of `.stdout` and `.stderr`
files, see example echo_1.stdout differences:

```
+# echo foobar abbacd
+
 foobar abbacd
```

it can be disabled and is disabled in this PR for some tests; use
`pg_bin.run_capture(..., with_command_header=False)` for that.

as a bonus this cleans up the echoed newlines from s3_scrubber
output which are also saved to file but echoed to test log.

Co-authored-by: Alexander Bayandin <alexander@neon.tech>

test_runner/fixtures/utils.py

@@ -49,7 +49,8 @@ def subprocess_capture(
     echo_stdout=False,
     capture_stdout=False,
     timeout=None,
-    **kwargs: Any,
+    with_command_header=True,
+    **popen_kwargs: Any,
 ) -> Tuple[str, Optional[str], int]:
     """Run a process and bifurcate its output to files and the `log` logger
 
@@ -86,13 +87,23 @@ def subprocess_capture(
             self.captured = ""
 
         def run(self):
+            first = with_command_header
             for line in self.in_file:
+                if first:
+                    # do this only after receiving any input so that we can
+                    # keep deleting empty files, or leave it out completly if
+                    # it was unwanted (using the file as input later for example)
+                    first = False
+                    # prefix the files with the command line so that we can
+                    # later understand which file is for what command
+                    self.out_file.write((f"# {' '.join(cmd)}\n\n").encode("utf-8"))
+
                 # Only bother decoding if we are going to do something more than stream to a file
                 if self.echo or self.capture:
                     string = line.decode(encoding="utf-8", errors="replace")
 
                     if self.echo:
-                        log.info(string)
+                        log.info(string.strip())
 
                     if self.capture:
                         self.captured += string
@@ -107,7 +118,7 @@ def subprocess_capture(
 
                 p = subprocess.Popen(
                     cmd,
-                    **kwargs,
+                    **popen_kwargs,
                     stdout=subprocess.PIPE,
                     stderr=subprocess.PIPE,
                 )