tests: Skip expensive speedup test/benchmark by default.

With this change, ‘make check’ runs in 230s on x86_64-linux-gnu (4-core
CPU) instead of 880s.

* tests/speedup.scm <top level>: Exit with 77 when
‘FIBERS_EXPENSIVE_TESTS’ is unset.

tests/speedup.scm

@@ -53,6 +53,14 @@
         (lp (1+ i) (make-vector (- words 2) #f))
         x)))
 
+(unless (getenv "FIBERS_EXPENSIVE_TESTS")
+  (newline (current-error-port))
+  (format (current-error-port) "Skipping expensive test/benchmark.~%")
+  (format (current-error-port) "\
+Set the 'FIBERS_EXPENSIVE_TESTS' environment variable to run it.~%")
+  (newline (current-error-port))
+  (exit 77))
+
 (measure-speedup
  (do-times 100000 (spawn-fiber (lambda () #t) #:parallel? #t)))
 (measure-speedup