REGTESTS: extend the default I/O timeouts and make them overridable

With the CI occasionally slowing down, we're starting to see again some
spurious failures despite the long 1-second timeouts. This reports false
positives that are disturbing and doesn't provide as much value as this
could. However at this delay it already becomes a pain for developers
to wait for the tests to complete.

This commit adds support for the new environment variable
HAPROXY_TEST_TIMEOUT that will allow anyone to modify the connect,
client and server timeouts. It was set to 5 seconds by default, which
should be plenty for quite some time in the CI. All relevant values
that were 200ms or above were replaced by this one. A few larger
values were left as they are special. One test for the set-timeout
action that used to rely on a fixed 1-sec value was extended to a
fixed 5-sec, as the timeout is normally not reached, but it needs
to be known to compare the old and new values.
diff --git a/reg-tests/README b/reg-tests/README
index 6993d97..ef721fd 100644
--- a/reg-tests/README
+++ b/reg-tests/README
@@ -63,3 +63,9 @@
       of the VTC file,
     - Put these files in a directory with the same name as the code area concerned
       by the bug ('peers', 'lua', 'acl' etc).
+
+Please note that most tests use a common set of timeouts defined by the
+environment variable HAPROXY_TEST_TIMEOUT. As much as possible, for regular I/O
+(i.e. not errors), please try to reuse that setting so that the value may
+easily be adjusted when running in some particularly slow environments, or be
+shortened to fail faster on developers' machines.