tests: make #testcase available as env var in test

It can be useful for e.g. helper functions to behave differently
depending on which test case is being executed. This patch helps with
that by making the #testcase case available as a TESTCASE environment
variable.

Differential Revision: https://phab.mercurial-scm.org/D1816

tests/run-tests.py

@@ -1334,6 +1334,9 b' class TTest(Test):'
             script.append(b'alias hg="%s"\n' % self._hgcommand)
         if os.getenv('MSYSTEM'):
             script.append(b'alias pwd="pwd -W"\n')
+        if self._case:
+            script.append(b'TESTCASE=%s\n' % shellquote(self._case))
+            script.append(b'export TESTCASE\n')
 
         n = 0
         for n, l in enumerate(lines):

tests/test-run-tests.t

@@ -1533,6 +1533,26 b' Test cases in .t files'
   python hash seed: * (glob)
   [1]
 
+Test TESTCASE variable
+
+  $ cat > test-cases-ab.t <<'EOF'
+  >   $ dostuff() {
+  >   >   echo "In case $TESTCASE"
+  >   > }
+  > #testcases A B
+  > #if A
+  >   $ dostuff
+  >   In case A
+  > #endif
+  > #if B
+  >   $ dostuff
+  >   In case B
+  > #endif
+  > EOF
+  $ rt test-cases-ab.t
+  ..
+  # Ran 2 tests, 0 skipped, 0 failed.
+
 Test automatic pattern replacement
 
   $ cat << EOF >> common-pattern.py