runtests: add %repeat[]% for test files

... and use this new keywords in all the test files larger than 50K to reduce
their sizes and make them a lot easier to read and understand.

Closes #6040

tests/FILEFORMAT.md

@@ -53,6 +53,21 @@ For example, to insert the binary octets 0, 1 and 255 into the test file:
 
     %hex[ %00%01%FF ]hex%
 
+## Repeat content
+
+In the preprocess stage, a special instruction can be used to have runtests.pl
+generate a repetetive sequence of bytes.
+
+To insert a sequence of repeat bytes, use this syntax to make the `<string>`
+get repeated `<number>` of times. The number has to be 1 or large and the
+string may contain `%HH` hexadecimal codes:
+
+    %repeat[<number> x <string>]%
+
+For example, to insert the word hello a 100 times:
+
+    %repeat[100 x hello]%
+
 # Variables
 
 When the test is preprocessed, a range of "variables" in the test file will be

tests/runtests.pl

@@ -3314,6 +3314,13 @@ sub subBase64 {
         $d =~ s/%([0-9A-Fa-f]{2})/chr(hex($1))/eg;
         $$thing =~ s/%%HEX%%/$d/;
     }
+    if($$thing =~ s/%repeat\[(\d+) x (.*)\]%/%%REPEAT%%/i) {
+        # decode %NN characters
+        my ($d, $n) = ($2, $1);
+        $d =~ s/%([0-9A-Fa-f]{2})/chr(hex($1))/eg;
+        my $all = $d x $n;
+        $$thing =~ s/%%REPEAT%%/$all/;
+    }
 }
 
 sub fixarray {