t: increase timeouts for slow commands with high load

We've seen many fails on ofborg, at lot of them ultimately appear to come down to
a timeout being hit, resulting in something like this:

Failure executing slapadd -F /<path>/slap.d -b dc=example -l /<path>/load.ldif.

Hopefully this resolves it for most cases.
I've done some endurance testing and this helps a lot.
some other commands also regularly time-out with high load:

- hydra-init
- hydra-create-user
- nix-store --delete

This should address most issues with tests randomly failing.

Used the following script for endurance testing:

```

import os
import subprocess

run_counter = 0
fail_counter = 0

while True:
    try:
        run_counter += 1
        print(f"Starting run {run_counter}")
        env = os.environ
        env["YATH_JOB_COUNT"] = "20"
        result = subprocess.run(["perl", "t/test.pl"], env=env)
        if (result.returncode != 0):
            fail_counter += 1
        print(f"Finish run {run_counter}, total fail count: {fail_counter}")
    except KeyboardInterrupt:
        print(f"Finished {run_counter} runs with {fail_counter} fails")
        break
```

In case someone else wants to do it on their system :).
Note that YATH_JOB_COUNT may need to be changed loosely based on your
cores.
I only have 4 cores (8 threads), so for others higher numbers might
yield better results in hashing out unstable tests.

t/queue-runner/build-locally-with-substitutable-path.t

@@ -39,7 +39,7 @@ subtest "Building, caching, and then garbage collecting the underlying job" => s
 
     ok(unlink(Hydra::Helper::Nix::gcRootFor($path)), "Unlinking the GC root for underlying Dependency succeeds");
 
-    (my $ret, my $stdout, my $stderr) = captureStdoutStderr(5, "nix-store", "--delete", $path);
+    (my $ret, my $stdout, my $stderr) = captureStdoutStderr(15, "nix-store", "--delete", $path);
     is($ret, 0, "Deleting the underlying dependency should succeed");
 };