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/data/test1294

@@ -44,7 +44,8 @@ HTTP GET with split initial request send
 CURL_SMALLREQSEND=128
 </setenv>
 <command>
-http://%HOSTIP:%HTTPPORT/%repeat[30 x 012345679]%/%TESTNUMBER -H "Among other public buildings in a certain town, which for many reasons it will be prudent to refrain from mentioning, and to which I will assign no fictitious name, there is one anciently common to most towns, great or small to ___, a workhouse; and in this workhouse was born; on a day and date which I need not trouble myself to repeat, inasmuch as it can be of no possible consequence to the reader, in this stage of the business at all events; the item of mortality whose name is prefixed to the head of this chapter: 511"
+http://%HOSTIP:%HTTPPORT/%repeat[30 x 012345679]%/%TESTNUMBER
+-H "Among other public buildings in a certain town, which for many reasons it will be prudent to refrain from mentioning, and to which I will assign no fictitious name, there is one anciently common to most towns, great or small to ___, a workhouse; and in this workhouse was born; on a day and date which I need not trouble myself to repeat, inasmuch as it can be of no possible consequence to the reader, in this stage of the business at all events; the item of mortality whose name is prefixed to the head of this chapter: 511"
 </command>
 </client>