tests: increase timeout for slow test

Test case `test-sparse-revlog.t` need some artifact (a bundle) build before it
can run. The artifact is expensive to build, but can be reused from one run to
the other. We are about to update that test to make the artifact building
automatic if `--allow-slow-tests` is passed. However, we need a bump the
timeout a bit to make sure the artifact building as time to finish.

We could maybe teach run-tests.py how to directly handle such artifacts.
However since there is only one of them for now, this seems premature.

There are also some room to speed up the bundle creation for
test-sparse-revlog.t

tests/run-tests.py

@@ -290,7 +290,7 b" if 'java' in sys.platform:"
 defaults = {
     'jobs': ('HGTEST_JOBS', multiprocessing.cpu_count()),
     'timeout': ('HGTEST_TIMEOUT', 180),
-    'slowtimeout': ('HGTEST_SLOWTIMEOUT', 500),
+    'slowtimeout': ('HGTEST_SLOWTIMEOUT', 1500),
     'port': ('HGTEST_PORT', 20059),
     'shell': ('HGTEST_SHELL', 'sh'),
 }