run-tests: $TESTDIR can be something else than $PWD

$TESTDIR is expected to be the directory were the test lives, and is often used
to reference helper functions. However, it is now set to $PWD at test invocation
time, so if `run-tests.py` is called from another folder, the test will fail.

The solution is to force $TESTDIR to be the base directory of the test itself,
irrespective of where the runner is called from.

tests/run-tests.py

@@ -2353,6 +2353,12 b' class TestRunner(object):'
 
         self._testdir = osenvironb[b'TESTDIR'] = getattr(
             os, 'getcwdb', os.getcwd)()
+        # assume all tests in same folder for now
+        if testdescs:
+            pathname = os.path.dirname(testdescs[0]['path'])
+            if pathname and not osenvironb[b'TESTDIR'].endswith('/'):
+                osenvironb[b'TESTDIR'] += '/'
+            osenvironb[b'TESTDIR'] += pathname
         if self.options.outputdir:
             self._outputdir = canonpath(_bytespath(self.options.outputdir))
         else:

tests/test-run-tests.t

@@ -1261,6 +1261,22 b' support for running a test outside the c'
   .
   # Ran 1 tests, 0 skipped, 0 failed.
 
+support for running run-tests.py from another directory
+  $ mkdir tmp && cd tmp
+  $ cat > useful-file.sh << EOF
+  > important command
+  > EOF
+
+  $ cat > test-folder.t << EOF
+  >   $ cat \$TESTDIR/useful-file.sh
+  >   important command
+  > EOF
+
+  $ cd ..
+  $ $PYTHON $TESTDIR/run-tests.py tmp/test-folder.t
+  .
+  # Ran 1 tests, 0 skipped, 0 failed.
+
 support for bisecting failed tests automatically
   $ hg init bisect
   $ cd bisect