Cut test suite runtime in binary roundtrips and integer sweeps (#5519)

* Cut test suite runtime in binary roundtrips and integer sweeps

The Linux CI jobs pass --no-skip, so skip() does not help there.
Parse each corpus file once in the binary roundtrip loops instead of
four times. Sample the 16-bit integer ranges with stride 7 (still hits
every low byte) and always keep the endpoints.

Also drop the 5M-node parse test to 500k, which still covers the
non-recursive destructor, and move jeopardy.json into its own skipped
test so the cheaper binary-format size checks actually run.

See #5418.

Signed-off-by: ayush-singh-0601 <singhayush062006@gmail.com>

* Drop useless int32_t casts in the sampled integer loops

ci_test_gcc compiles with -Werror=useless-cast. On that compiler
int32_t is int, so static_cast<int32_t> of the loop bound is an
error. The bounds are already int, and the sampled values do not
change.

Signed-off-by: ayush-singh-0601 <singhayush062006@gmail.com>

* Revert unit-binary_formats.cpp to develop and fix comment

Revert tests/src/unit-binary_formats.cpp to its develop state.
The test-case split made valgrind jobs slower instead of faster,
because the cheaper corpus files (canada/twitter/citm/sample)
now ran under valgrind where they never did before.

Fix the next_integer_sample comment: the function has no 'first'
parameter, so describe what the function actually does.

Signed-off-by: ayush-singh-0601 <singhayush062006@gmail.com>

---------

Signed-off-by: ayush-singh-0601 <singhayush062006@gmail.com>

tests/src/test_utils.hpp

@@ -9,6 +9,7 @@
 #pragma once
 
 #include <cstdint> // uint8_t
+#include <cstddef> // size_t
 #include <fstream> // ifstream, istreambuf_iterator, ios
 #include <vector> // vector
 
@@ -24,6 +25,23 @@ namespace utils
 template<typename T>
 inline void ignore_return_value(T&& /*unused*/) noexcept {}
 
+// Advance i toward last (inclusive) by stride, always visiting last.
+// stride 7 is coprime to 256, so every low-byte residue is still hit.
+template<typename T>
+T next_integer_sample(T i, T last, T stride)
+{
+    if (i >= last)
+    {
+        return static_cast<T>(last + 1);
+    }
+    if (stride > 0 && i > static_cast<T>(last - stride))
+    {
+        return last;
+    }
+    const T n = static_cast<T>(i + stride);
+    return n < last ? n : last;
+}
+
 inline std::vector<std::uint8_t> read_binary_file(const std::string& filename)
 {
     std::ifstream file(filename, std::ios::binary);