tests: better testing of loaded certificates

Tests were failing on systems like RHEL 7 where loading the system
certificates results in CA certs being reported to Python. We add
a feature that detects when we're able to load *and detect* the
loading of system certificates. We update the tests to cover the
3 scenarios:

1) system CAs are loadable and detected
2) system CAs are loadable but not detected
3) system CAs aren't loadable

tests/hghave.py

@@ -418,6 +418,25 b' def has_defaultcacerts():'
     from mercurial import sslutil
     return sslutil._defaultcacerts() or sslutil._canloaddefaultcerts
 
+@check("defaultcacertsloaded", "detected presence of loaded system CA certs")
+def has_defaultcacertsloaded():
+    import ssl
+    from mercurial import sslutil
+
+    if not has_defaultcacerts():
+        return False
+    if not has_sslcontext():
+        return False
+
+    cafile = sslutil._defaultcacerts()
+    ctx = ssl.create_default_context()
+    if cafile:
+        ctx.load_verify_locations(cafile=cafile)
+    else:
+        ctx.load_default_certs()
+
+    return len(ctx.get_ca_certs()) > 0
+
 @check("windows", "Windows")
 def has_windows():
     return os.name == 'nt'

tests/test-https.t

@@ -47,12 +47,26 b' Test server address cannot be reused'
 Our test cert is not signed by a trusted CA. It should fail to verify if
 we are able to load CA certs.
 
-#if defaultcacerts
+#if sslcontext defaultcacerts no-defaultcacertsloaded
   $ hg clone https://localhost:$HGPORT/ copy-pull
   (an attempt was made to load CA certificates but none were loaded; see https://mercurial-scm.org/wiki/SecureConnections for how to configure Mercurial to avoid this error)
   abort: error: *certificate verify failed* (glob)
   [255]
-#else
+#endif
+
+#if no-sslcontext defaultcacerts
+  $ hg clone https://localhost:$HGPORT/ copy-pull
+  abort: error: *certificate verify failed* (glob)
+  [255]
+#endif
+
+#if defaultcacertsloaded
+  $ hg clone https://localhost:$HGPORT/ copy-pull
+  abort: error: *certificate verify failed* (glob)
+  [255]
+#endif
+
+#if no-defaultcacerts
   $ hg clone https://localhost:$HGPORT/ copy-pull
   abort: localhost certificate error: no certificate received
   (set hostsecurity.localhost:certfingerprints=sha256:62:09:97:2f:97:60:e3:65:8f:12:5d:78:9e:35:a1:36:7a:65:4b:0e:9f:ac:db:c3:bc:6e:b6:a3:c0:16:e0:30 config setting or use --insecure to connect insecurely)

tests/test-patchbomb-tls.t

@@ -43,7 +43,7 b' Utility functions:'
 Our test cert is not signed by a trusted CA. It should fail to verify if
 we are able to load CA certs:
 
-#if defaultcacerts
+#if sslcontext defaultcacerts no-defaultcacertsloaded
   $ try
   this patch series consists of 1 patches.
   
@@ -53,6 +53,35 b' we are able to load CA certs:'
   [255]
 #endif
 
+#if no-sslcontext defaultcacerts
+  $ try
+  this patch series consists of 1 patches.
+  
+  
+  (?i)abort: .*?certificate.verify.failed.* (re)
+  [255]
+#endif
+
+#if defaultcacertsloaded
+  $ try
+  this patch series consists of 1 patches.
+  
+  
+  (?i)abort: .*?certificate.verify.failed.* (re)
+  [255]
+
+#endif
+
+#if no-defaultcacerts
+  $ try
+  this patch series consists of 1 patches.
+  
+  
+  abort: localhost certificate error: no certificate received
+  (set hostsecurity.localhost:certfingerprints=sha256:62:09:97:2f:97:60:e3:65:8f:12:5d:78:9e:35:a1:36:7a:65:4b:0e:9f:ac:db:c3:bc:6e:b6:a3:c0:16:e0:30 config setting or use --insecure to connect insecurely)
+  [255]
+#endif
+
   $ DISABLECACERTS="--config devel.disableloaddefaultcerts=true"
 
 Without certificates: