runtests: allow client/command to span multiple lines, and use it

Some curl command-lines are long, often repetitive, and difficult
to read or write:

Before this patch (1 test == 1 line):
- <=78 characters: 1099 tests
- 79-132 characters: 500 tests
- 133+ characters: 217 tests: patch meant to help with some of these.

After this patch:
- <=78 characters: 1288 lines
- 79-132 characters: 526 lines
- 133+ characters: 190 lines

After this patch it's possible to fold long lines into multiple ones.
Folding can reduce greppability, thus this is primarily useful for cases
when the options are repetitive, e.g. a list of form options, headers,
mail parameters and the like.

Closes #19500

docs/tests/FILEFORMAT.md

@@ -579,6 +579,9 @@ deleted.
 ### `<command [option="no-q/no-output/no-include/force-output/binary-trace"] [timeout="secs"][delay="secs"][type="perl/shell"]>`
 Command line to run.
 
+If the command spans multiple lines, they are concatenated with a space added
+between them.
+
 Note that the URL that gets passed to the server actually controls what data
 that is returned. The last slash in the URL must be followed by a number. That
 number (N) is used by the test-server to load test case N and return the data