test: further limit the concurrency of WPTRunner for RISC-V

Even when the concurrency of WPTRunner is limited to 10, flaky WPT tests
are still observed on RISC-V with Sv39.

For example, test/wpt/test-compression.js failed in
https://github.com/riscv-forks/node-riscv/actions/runs/27756747452 and
https://github.com/riscv-forks/node-riscv/actions/runs/27544719447,
where the cause of the failure is a failing `mmap` of roughly 8GiB:

    [pid 954955] mmap(NULL, 8589996032, PROT_NONE, ...

Further limit the concurrency of WPTRunner to fix flaky tests.
See #60591 for the reason of limiting the WPT test concurrency on
RISC-V.

Signed-off-by: Levi Zim <rsworktech@outlook.com>

Author: kxxt

test/common/wpt.js

@@ -563,7 +563,7 @@ class WPTRunner {
     // memories(27 from a fresh node repl). Limit the concurrency to avoid
     // creating too many wasm memories that would fail.
     if (process.arch === 'riscv64' || process.arch === 'riscv32') {
-      concurrency = Math.min(10, concurrency);
+      concurrency = Math.min(5, concurrency);
     }
 
     this.path = path;