test: Change default port for httpcore tests

The code previously used 9090 which is memorable enough to conflict
with other code that could be running on developer machines.
New default of 46406 is unlikely to hit such conflicts.

Also, some small improvements in the httpcore bats script.

Author: dividedmind

agent/test/httpcore/httpcore.bats

@@ -8,12 +8,14 @@
 load '../helper'
 
 setup_file() {
-mkdir -p build/log
+  agent_root="$BATS_TEST_DIRNAME/../.."
+  mkdir -p "$agent_root/build/log"
 
-export LOG="$(getcwd)/build/log/httpcore.log"
-export SERVER_PORT=9090
-export WS_URL=${WS_URL:-http://localhost:9090}
-  cd test/httpcore
+  export LOG="$agent_root/build/log/httpcore.log"
+  export SERVER_PORT=46406
+  export WS_URL=${WS_URL:-http://localhost:${SERVER_PORT}}
+
+  cd "$BATS_TEST_DIRNAME" || exit
   _configure_logging
 
   printf 'getting set up' >&3