#
# Recommended minimum configuration:
#

# Example rule allowing access from your local networks.
# Adapt to list your (internal) IP networks from where browsing
# should be allowed
acl localnet src 10.0.0.0/8   # RFC1918 possible internal network
acl localnet src 172.16.0.0/12   # RFC1918 possible internal network
acl localnet src 192.168.0.0/16   # RFC1918 possible internal network
acl localnet src fc00::/7       # RFC 4193 local private network range
acl localnet src fe80::/10      # RFC 4291 link-local (directly plugged) machines

acl SSL_ports port 443
acl Safe_ports port 80      # http
acl Safe_ports port 21      # ftp
acl Safe_ports port 443      # https
acl Safe_ports port 70      # gopher
acl Safe_ports port 210      # wais
acl Safe_ports port 1025-65535   # unregistered ports
acl Safe_ports port 280      # http-mgmt
acl Safe_ports port 488      # gss-http
acl Safe_ports port 591      # filemaker
acl Safe_ports port 777      # multiling http
acl CONNECT method CONNECT

#
# Recommended minimum Access Permission configuration:
#
# Deny requests to certain unsafe ports
http_access deny !Safe_ports

# Deny CONNECT to other than secure SSL ports
http_access deny CONNECT !SSL_ports

# Only allow cachemgr access from localhost
http_access allow localhost manager
http_access deny manager

# We strongly recommend the following be uncommented to protect innocent
# web applications running on the proxy server who think the only
# one who can access services on "localhost" is a local user
#http_access deny to_localhost

#
# INSERT YOUR OWN RULE(S) HERE TO ALLOW ACCESS FROM YOUR CLIENTS
#

# Example rule allowing access from your local networks.
# Adapt localnet in the ACL section to list your (internal) IP networks
# from where browsing should be allowed
http_access allow localnet
http_access allow localhost

# And finally deny all other access to this proxy
http_access deny all

# Squid normally listens to port 3128
http_port 3128

# Uncomment and adjust the following to add a disk cache directory.
#cache_dir ufs /var/cache/squid 100 16 256

# Leave coredumps in the first cache dir
coredump_dir /var/cache/squid

#
# Add any of your own refresh_pattern entries above these.
#
refresh_pattern ^ftp:      1440   20%   10080
refresh_pattern ^gopher:   1440   0%   1440
refresh_pattern -i (/cgi-bin/|\?) 0   0%   0
refresh_pattern .      0   20%   4320

[08:08] Testing Started.
   Proxy Server
   Address:   xxx.xxx.xxx.xxx:3128
   Protocol:   HTTPS
   Authentication: NO

[08:08] Starting: Test 1: Connection to the Proxy Server
[08:08] IP Address: xxx.xxx.xxx.xxx
[08:08] Connection established
[08:08] Test passed.
[08:08] Starting: Test 2: Connection through the Proxy Server
   Error: the proxy server (Squid) is not configured to allow SSL tunnel to port 80.
   To fix the problem please find and comment the following line in the Squid
   configuration file (squid.conf):
      http_access deny CONNECT !SSL_ports
   The proxy server reply header is:
      HTTP/1.1 403 Forbidden
      Server: squid/3.5.28
      Mime-Version: 1.0
      Date: Tue, 11 Sep 2018 07:08:07 GMT
      Content-Type: text/html;charset=utf-8
      Content-Length: 3342
      X-Squid-Error: ERR_ACCESS_DENIED 0
      Vary: Accept-Language
      Content-Language: en
      X-Cache: MISS from DESKTOP-G0C5A5M
      Via: 1.1 DESKTOP-G0C5A5M (squid/3.5.28)
      Connection: keep-alive
[08:08] Test failed.
[08:08] Testing Finished.

Chico no se que parte del log no entiendes, aquí dice que te estas conectando por el puerto 80 por https y que el proxy (squid) solo tiene configurado el 443 para https y por tanto te dice que se va a conectar por ese puerto (80) y con ese protocolo (https) la madre del topo.

  To fix the problem please find and comment the following line in the Squid
   configuration file (squid.conf):
      http_access deny CONNECT !SSL_ports

El log te esta diciendo que anules la regla http_access deny CONNECT !SSL_ports comentándola.
Ponle un # al inicio.