tests: remove assertion for '!uncertain' on .txt
tests: remove assertion for '!uncertain' on .txt

Virtaal installs a mime package for various .po-like file formats, one
of which has the extension .txt.  This causes GLib to report ".txt"
files still as "text/plain" but no longer with complete certainty.

The result is that asserting !uncertain during the testsuite causes the
test to fail if Virtaal happens to be installed.

Remove this assertion.

gale320/glib,tchakabam/glib,MathieuDuponchelle/glib,lukasz-skalski/glib,johne53/MB3Glib,krichter722/glib,tamaskenez/glib,lukasz-skalski/glib,johne53/MB3Glib,gale320/glib,lukasz-skalski/glib,mzabaluev/glib,tamaskenez/glib,tamaskenez/glib,mzabaluev/glib,tchakabam/glib,cention-sany/glib,cosimoc/glib,gale320/glib,cention-sany/glib,cosimoc/glib,krichter722/glib,MathieuDuponchelle/glib,ieei/glib,cosimoc/glib,Distrotech/glib,mzabaluev/glib,cosimoc/glib,ieei/glib,mzabaluev/glib,tchakabam/glib,MathieuDuponchelle/glib,Distrotech/glib,cosimoc/glib,MathieuDuponchelle/glib,endlessm/glib,endlessm/glib,endlessm/glib,krichter722/glib,MathieuDuponchelle/glib,johne53/MB3Glib,Distrotech/glib,tamaskenez/glib,Distrotech/glib,tchakabam/glib,cention-sany/glib,krichter722/glib,johne53/MB3Glib,tchakabam/glib,ieei/glib,gale320/glib,lukasz-skalski/glib,lukasz-skalski/glib,ieei/glib,gale320/glib,tamaskenez/glib,johne53/MB3Glib,Distrotech/glib,krichter722/glib,cention-sany/glib,johne53/MB3Glib,mzabaluev/glib,endlessm/glib,cention-sany/glib,ieei/glib,endlessm/glib