tests: disabled 1903 now

Test 1903 is doing HTTP pipelining, and that is a timing and ordering
sensitive operation and this fails far too often on the Travis CI
leading to people more or less ignoring test failures there. Not good.

The end of pipelning is probably coming sooner rather than later
anyway...
diff --git a/tests/data/DISABLED b/tests/data/DISABLED
index 50544ab..6ab4c43 100644
--- a/tests/data/DISABLED
+++ b/tests/data/DISABLED
@@ -16,3 +16,5 @@
 # test 1510 casues problems on the CI on github
 # example: https://travis-ci.org/curl/curl/builds/81633600
 1510
+# Pipelining test that is causing false positives a little too often
+1903