test: enable flaky tests on FreeBSD 12

Testing on FreeBSD 12 had some tests previously blocked to avoid of
flaky fails. For now we have the ability to avoid of it in test-run
using checksums for fails with opened issues. So adding back 7 tests
to testing on FreeBSD 12.

Closes #4271

test/app-tap/http_client.skipcond

 import platform
 
-# Disabled on FreeBSD due to flaky fail #4271.
-if platform.system() == 'FreeBSD':
-    self.skip = 1
-
 # Disabled on OpenBSD due to fail #XXXX.
 if platform.system() == 'OpenBSD':
     self.skip = 1

test/app/crypto.skipcond

 import platform
 
-# Disabled on FreeBSD due to flaky fail #4271.
-if platform.system() == 'FreeBSD':
-    self.skip = 1
-
 # Disabled on OpenBSD due to fail #XXXX.
 if platform.system() == 'OpenBSD':
     self.skip = 1

test/app/digest.skipcond

 import platform
 
-# Disabled on FreeBSD due to flaky fail #4271.
-if platform.system() == 'FreeBSD':
-    self.skip = 1
-
 # Disabled on OpenBSD due to fail #XXXX.
 if platform.system() == 'OpenBSD':
     self.skip = 1

test/box-tap/cfg.skipcond

 import platform
 
-# Disabled on FreeBSD due to fail #4271:
-#   Data segment size exceeds process limit
-if platform.system() == 'FreeBSD':
-    self.skip = 1
-
 # Disabled on OpenBSD due to fail #XXXX:
 if platform.system() == 'OpenBSD':
     self.skip = 1

test/box/net_msg_max.skipcond

@@ -4,8 +4,4 @@ import platform
 if platform.system() == 'OpenBSD':
     self.skip = 1
 
-# Disabled on FreeBSD due to flaky fail #4271.
-if platform.system() == 'FreeBSD':
-    self.skip = 1
-
 # vim: set ft=python:

test/replication/skip_conflict_row.skipcond

-import platform
-
-# Disabled on FreeBSD due to flaky fail #4271.
-if platform.system() == 'FreeBSD':
-    self.skip = 1
-
-# vim: set ft=python:

test/wal_off/snapshot_stress.skipcond

 import platform
 
-# Disabled on FreeBSD due to flaky fail #4271.
-if platform.system() == 'FreeBSD':
-    self.skip = 1
-
 # Disabled on OpenBSD due to fail #XXXX.
 if platform.system() == 'OpenBSD':
     self.skip = 1