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

tests/runner.pm

@@ -804,10 +804,13 @@ sub singletest_run {
     my ($testnum, $testtimings) = @_;
 
     # get the command line options to use
-    my ($cmd, @blaha)= getpart("client", "command");
-    if($cmd) {
-        # make some nice replace operations
+    my $cmd;
+    my @cmd = getpart("client", "command");
+    if(@cmd) {
+        # allow splitting the command-line to multiple lines
+        $cmd = join(' ', @cmd);
         $cmd =~ s/\n//g; # no newlines please
+        chomp $cmd; # no newlines please
         # substitute variables in the command line
     }
     else {