testsuite: bump PartialDownSweep timeout to 5x on wasm32

(cherry picked from commit b1e0c313)

testsuite/tests/ghc-api/downsweep/all.T

-setTestOpts(when(arch('wasm32'), run_timeout_multiplier(2)))
+setTestOpts(when(arch('wasm32'), run_timeout_multiplier(5)))
 
 test('PartialDownsweep',
      [ extra_run_opts('"' + config.libdir + '"')