gotest: increase the test timeout

The default test timeout duration of the gc compiler is 10 minutes,
and the current default timeout duration of gofrontend is 240 seconds,
which is not long enough for some big tests. This CL changes it to
600s, so that all tests have enough time to complete.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/229657

gcc/go/gofrontend/MERGE

-b76c83f34c006938fe6c3311d949496990bc5db9
+761d68dacefc578e45ff299761f20989aef67823
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/testsuite/gotest

@@ -36,7 +36,7 @@ keep=false
 pkgpath=
 prefix=
 dejagnu=no
-timeout=240
+timeout=600
 testname=""
 bench=""
 trace=false