REGTESTS: use HAPROXY_TEST_TIMEOUT for wait in purge srv/be

REGTESTS: use HAPROXY_TEST_TIMEOUT for wait in purge srv/be

Replace hardcoded value for wait timeout in regtests for server/backend
purge mechanism. The variable HAPROXY_TEST_TIMEOUT is used with 5s as a
default value if unspecified, similar to what is already used for
haproxy timeout in most of the test suite.

This should prevent spurrious test failures due to timing issue. In
particular, this should fix github CI runners execution.

Some timers remain hardcoded to small values when wait is expected to
either fail or suceed immediately. A larger value would cause the test
to take an unnecessary delay when a real failure should be reported.