Makefile: allow setting HGTESTFLAGS in shell environment for TESTFLAGS

I keep wanting to run 'make tests', but I forget to set TESTFLAGS='-j
16' or whatever is reasonable for my machine. This lets me just set it
once in my shell settings and forget it.

Makefile

@@ -14,6 +14,7 b' PYFILES:=$(shell find mercurial hgext do'
 DOCFILES=mercurial/help/*.txt
 export LANGUAGE=C
 export LC_ALL=C
+TESTFLAGS ?= $(shell echo $$HGTESTFLAGS)
 
 # Set this to e.g. "mingw32" to use a non-default compiler.
 COMPILER=