tests: fix some http/2 tests for older versions of nghttpx

- Add regex that strips http/2 server header name to those http/2 tests
  that don't already have it.

- Improve that regex in all http/2 tests.

Tests 358 and 359 were failing for me before this change on a system
that uses an older version of nghttpx which includes its version number
in the server header.

Closes https://github.com/curl/curl/pull/6139

tests/data/test1701

@@ -71,13 +71,12 @@ accept-ranges: bytes
 content-length: 6
 content-type: text/html
 funny-head: yesyes
-server: cut-out
 via: 1.1 nghttpx
 
 -foo-
 </stdout>
 <stripfile>
-s/^server:.*/server: cut-out
/
+s/^server: nghttpx.*\r?\n//
 </stripfile>
 </verify>
 </testcase>