minor: rename pytest fixture
minor: rename pytest fixture

tctx clashes with an omnipresent fixture on sans-io and my IDE
is not smart enough to figure out which one is which.

Kriechi/mitmproxy,vhaupert/mitmproxy,mitmproxy/mitmproxy,mhils/mitmproxy,vhaupert/mitmproxy,mitmproxy/mitmproxy,vhaupert/mitmproxy,Kriechi/mitmproxy,mitmproxy/mitmproxy,Kriechi/mitmproxy,mhils/mitmproxy,mitmproxy/mitmproxy,mhils/mitmproxy,mhils/mitmproxy,mitmproxy/mitmproxy,vhaupert/mitmproxy,mhils/mitmproxy,Kriechi/mitmproxy