dev-python/guppy3: fix test to cd to "${T}" first

Doesn't seem necessary at first but according to the guide [1] it
could cause troubles if it's not enabled. Bugs have been observed
in upsteam CI indeed when Python sees two copies of the source [2].

[1] https://projects.gentoo.org/python/guide/test.html#importerrors-for-c-extensions
[2] https://github.com/zhuyifei1999/guppy3/issues/37

Signed-off-by: YiFei Zhu <zhuyifei1999@gmail.com>

dev-python/guppy3/guppy3-3.1.3.ebuild

@@ -19,5 +19,6 @@ SLOT="0"
 KEYWORDS="~amd64 ~x86"
 
 python_test() {
-	"${EPYTHON}" guppy/heapy/test/test_all.py || die "tests failed"
+	cd "${T}" || die
+	"${EPYTHON}" "${S}"/guppy/heapy/test/test_all.py || die
 }