Fix TESTPYTHON path for out-of-tree builds

Summary:
This use of `$(PWD)`, from D50354346, isn't correct: it inherits
whatever `$PWD` happened to be in the calling shell. If you use an out-of-tree
build and run `make -C foo/...`, `$PWD` won't point at `foo/...`. `$CURDIR` is
set by `make` and provides what we want.

Reviewed By: mpage

Differential Revision: D52547938

fbshipit-source-id: 1e0edcafcd2fb5362419c12fae82137c6d826622

Makefile.pre.in

@@ -1646,7 +1646,7 @@ $(CINDERX_OBJS): $(CINDERX_PRIVATE_HEADERS) $(CINDERX_PUBLIC_HEADERS) $(PYTHON_H
 ######################################################################
 
 TESTOPTS=	$(EXTRATESTOPTS)
-TESTPYTHON=	$(RUNSHARED) $(PWD)/$(BUILDPYTHON) $(TESTPYTHONOPTS)
+TESTPYTHON=	$(RUNSHARED) $(CURDIR)/$(BUILDPYTHON) $(TESTPYTHONOPTS)
 TESTRUNNER=	$(TESTPYTHON) -X usepycompiler $(srcdir)/Tools/scripts/run_tests.py
 TESTRUNNERLAZYIMPORTS=	$(TESTPYTHON) -L $(srcdir)/Tools/scripts/run_tests.py
 TESTTIMEOUT=	1200