tests: improve reliability of TFTP tests

Stop checking the timeout used by the client under test (for most
tests). The timeout will change if the TFTP test server is slow (such as
happens on an overprovisioned CI server) because the client will retry
and reduce its timeout, and the actual value is not important for most
tests.

test285 is changed a different way, by increasing the connect timeout.
This improves test coverage by allowing the changed timeout value to be
checked, but improves reliability with a carefully-chosen timeout that
not only allows twice the time to respond as before, but also allows
several retries before the client will change its timeout value.

Ref: #11328

tests/data/test2003

@@ -72,6 +72,9 @@ moo
 #
 # Verify data after the test has been "shot"
 <verify>
+<strip>
+^timeout = [5-6]$
+</strip>
 <protocol>
 GET /%TESTNUMBER0001 HTTP/1.1
 Host: %HOSTIP:%HTTPPORT
@@ -89,13 +92,11 @@ opcode = 1
 mode = octet
 tsize = 0
 blksize = 512
-timeout = 6
 filename = /%TESTNUMBER0003
 opcode = 1
 mode = octet
 tsize = 0
 blksize = 512
-timeout = 6
 filename = /%TESTNUMBER0003
 EPSV
 SIZE %TESTNUMBER0002