test-run-test: use explicit variables to clarify paths

Since $TESTTMP and $TESTDIR are defined for both the main scope and the test
run during the test, the situation get confusing. To work around that, we
define some new, explicit, variables with the values we needs to keep track
of.

Differential Revision: https://phab.mercurial-scm.org/D11034

tests/test-run-tests.t

@@ -1381,8 +1381,28 b' Add support for external test formatter'
 Test reusability for third party tools
 ======================================
 
-  $ mkdir "$TESTTMP"/anothertests
-  $ cd "$TESTTMP"/anothertests
+  $ THISTESTDIR="$TESTDIR"
+  $ export THISTESTDIR
+  $ THISTESTTMP="$TESTTMP"
+  $ export THISTESTTMP
+
+#if windows
+
+  $ NEWTESTDIR="$THISTESTTMP"\\anothertests
+
+#else
+
+  $ NEWTESTDIR="$THISTESTTMP"/anothertests
+
+#endif
+
+  $ export NEWTESTDIR
+
+  $ echo creating some new test in: $NEWTESTDIR
+  creating some new test in: $TESTTMP\anothertests (windows !)
+  creating some new test in: $TESTTMP/anothertests (no-windows !)
+  $ mkdir "$NEWTESTDIR"
+  $ cd "$NEWTESTDIR"
 
 test that `run-tests.py` can execute hghave, even if it runs not in
 Mercurial source tree.
@@ -1400,22 +1420,20 b' Mercurial source tree.'
 test that RUNTESTDIR refers the directory, in which `run-tests.py` now
 running is placed.
 
+
   $ cat > test-runtestdir.t <<EOF
-  > - $TESTDIR, in which test-run-tests.t is placed
-  > - \$TESTDIR, in which test-runtestdir.t is placed (expanded at runtime)
-  > - \$RUNTESTDIR, in which run-tests.py is placed (expanded at runtime)
+  > # \$THISTESTDIR, in which test-run-tests.t (this test file) is placed
+  > # \$THISTESTTMP, in which test-run-tests.t (this test file) is placed
+  > # \$TESTDIR, in which test-runtestdir.t is placed (expanded at runtime)
+  > # \$RUNTESTDIR, in which run-tests.py is placed (expanded at runtime)
   > 
-  > #if windows
-  >   $ test "\$TESTDIR" = "$TESTTMP\\anothertests"
-  > #else
-  >   $ test "\$TESTDIR" = "$TESTTMP"/anothertests
-  > #endif
+  >   $ test "\$TESTDIR" = "\$NEWTESTDIR"
   > If this prints a path, that means RUNTESTDIR didn't equal
-  > TESTDIR as it should have.
-  >   $ test "\$RUNTESTDIR" = "$TESTDIR" || echo "\$RUNTESTDIR"
+  > THISTESTDIR as it should have.
+  >   $ test "\$RUNTESTDIR" = "\$THISTESTDIR" || echo "\$RUNTESTDIR"
   > This should print the start of check-code. If this passes but the
   > previous check failed, that means we found a copy of check-code at whatever
-  > RUNTESTSDIR ended up containing, even though it doesn't match TESTDIR.
+  > RUNTESTSDIR ended up containing, even though it doesn't match THISTESTDIR.
   >   $ head -n 3 "\$RUNTESTDIR"/../contrib/check-code.py | sed 's@.!.*python3@#!USRBINENVPY@'
   >   #!USRBINENVPY
   >   #