Limit the number of threads for subprocess tests

Tests created using std.test.Tests.fork now limit the number of Inko
threads the subprocess uses, reducing the chances of such tests reaching
thread count limits and thus crashing the test.

Changelog: fixed

std/src/std/test.inko

@@ -394,6 +394,13 @@ class pub Process {
     cmd.stdout = Stream.Piped
     cmd.stderr = Stream.Piped
     cmd.variables.set(CHILD_VAR, id.to_string)
+
+    # Inko uses a set of thread pools for running processes, with their sizes
+    # depending on the number of CPU cores. We limit the number of threads here
+    # so we don't overload the system when running sub process tests on systems
+    # with many CPU cores.
+    cmd.variables.set('INKO_PROCESS_THREADS', '1')
+    cmd.variables.set('INKO_BACKUP_THREADS', '1')
     Process(cmd: cmd, stdin: '')
   }