tests: rename from test-config-case.t to test-config.t for centralization

Before this patch, there is no test script testing configuration
handling generally. "test-config-case.t" seems to be specific for
testing case sensitive configuration.

This patch renames from "test-config-case.t" to "test-config.t" for
centralization of tests around configuration handling.

tests/test-config.t

@@ -1,11 +1,13 b''
 hide outer repo
   $ hg init
 
+Test case sensitive configuration
+
   $ echo '[Section]' >> $HGRCPATH
   $ echo 'KeY = Case Sensitive' >> $HGRCPATH
   $ echo 'key = lower case' >> $HGRCPATH
 
   $ hg showconfig Section
   Section.KeY=Case Sensitive
   Section.key=lower case