REGTESTS: sample_fetches: add a test for the constant and process fetches

REGTESTS: sample_fetches: add a test for the constant and process fetches

The constant fetches always_true, always_false, str, int, bin, bool,
ipv4, ipv6 and meth, the process state ones pid, thread, tgroup, nbproc,
quic_enabled, cpu_calls, cpu_ns_tot, cpu_ns_avg, lat_ns_tot, lat_ns_avg,
cpu_usage_proc, cpu_usage_thr and cpu_usage_grp, plus date_us, env(),
rand() and dump_all_vars had little or no coverage.

The values which cannot be predicted are matched against their expected
shape, while pid is compared to the process identifier reported by
vtest, rand(10) must stay within its range, env() is looked up on the
HAPROXY_LOCALPEER variable which the process always exports, and
dump_all_vars must mention a variable set by an earlier rule.