tdf#165503: sc_subsequent_export_test4: Use precise XPath

The previous one was working fine, too, but it was missing the
last, single element in the path.

Change-Id: I8d8d359436f52fd8d5397b29905747e40d5f8793
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/182550
Tested-by: Jenkins
Reviewed-by: Aron Budea <aron.budea@collabora.com>

sc/qa/unit/subsequent_export_test4.cxx

@@ -2176,12 +2176,12 @@ CPPUNIT_TEST_FIXTURE(ScExportTest4, testTdf165503)
     // The textual date output can depend on locale, but it'll differ from expected value either way
     assertXPathContent(pChart1,
                        "/c:chartSpace/c:chart/c:plotArea/c:lineChart/c:ser/c:cat/c:numRef/"
-                       "c:numCache/c:pt[@idx=\"0\"]",
+                       "c:numCache/c:pt[@idx=\"0\"]/c:v",
                        u"44199");
     // And similarly
     assertXPathContent(pChart1,
                        "/c:chartSpace/c:chart/c:plotArea/c:lineChart/c:ser/c:cat/c:numRef/"
-                       "c:numCache/c:pt[@idx=\"4\"]",
+                       "c:numCache/c:pt[@idx=\"4\"]/c:v",
                        u"44844");
 
     // There should be no node with idx 5 (cell is empty)