gotest: add LC_ALL=C

R=r
https://golang.org/cl/154067

src/cmd/gotest/gotest

@@ -11,6 +11,7 @@
 # Makes egrep,grep work better with · if we put them
 # in ordinary C mode instead of what the current language is.
 unset LANG
+export LC_ALL=C
 export LC_CTYPE=C
 
 _GC=$GC	# Make.$GOARCH will overwrite this