Use variables instead of substitutions

Using variables removes the ambiguity about when to use variables and
when to use substitutions. Variables always work. Substitutions only
work when Autoconf knows about them which is not always the case.

The variables are also placed between quotes to ensures spaces in the
variables are handled properly.

src/test/include.am

@@ -1,8 +1,8 @@
 AM_TESTS_ENVIRONMENT = \
-	export PYTHON=@PYTHON@; \
-	export SHELL=@SHELL@; \
-	export abs_top_srcdir=@abs_top_srcdir@; \
-	export builddir=@builddir@; \
+	export PYTHON="$(PYTHON)"; \
+	export SHELL="$(SHELL)"; \
+	export abs_top_srcdir="$(abs_top_srcdir)"; \
+	export builddir="$(builddir)"; \
 	export TESTING_TOR_BINARY="$(TESTING_TOR_BINARY)";
 
 TESTSCRIPTS = src/test/test_zero_length_keys.sh