build: write failures.txt, not failures.log

Everyone has good file associations set up for .txt, less so for .log.
In particular browsers will ask you to download it, which isn't usually
what you want.

Release note: None

build/teamcity-support.sh

@@ -28,7 +28,7 @@ run_counter=-1
 
 # Takes args that produce `go test -json` output. It filters stdout to contain
 # only test output related to failing tests and run/pass/skip events for the
-# other tests (no output). It writes artifacts/failures.log containing text
+# other tests (no output). It writes artifacts/failures.txt containing text
 # output for the failing tests.
 # It's valid to call this multiple times; all output artifacts will be
 # preserved.
@@ -78,8 +78,8 @@ function run_json_test() {
   fi
 
   tc_start_block "artifacts"
-  # Create (or append to) failures.log artifact and delete stripped.txt.
-  testfilter -mode=omit < artifacts/stripped.txt | testfilter -mode convert >> artifacts/failures.log
+  # Create (or append to) failures.txt artifact and delete stripped.txt.
+  testfilter -mode=omit < artifacts/stripped.txt | testfilter -mode convert >> artifacts/failures.txt
   rm -f artifacts/stripped.txt
 
   # Keep the debug file around for failed builds. Compress it to avoid