[context] Add path prefix/suffix manipulation.

recipe_modules/context/examples/full.expected/basic.json

Index: recipe_modules/context/examples/full.expected/basic.json
diff --git a/recipe_modules/context/examples/full.expected/basic.json b/recipe_modules/context/examples/full.expected/basic.json
index bf0aabdbb0cbabfd0cc26c863b28fee94ae77dc8..0da489f146602b26b8ab34cc4820eac4c525318e 100644
--- a/recipe_modules/context/examples/full.expected/basic.json
+++ b/recipe_modules/context/examples/full.expected/basic.json
@@ -18,6 +18,7 @@
   {
     "cmd": [
       "mkdir",
+      "-p",
       "subdir"
     ],
     "name": "mk subdir"
@@ -44,14 +45,35 @@
     "cmd": [
       "bash",
       "-c",
-      "echo $HELLO; echo $HOME"
+      "echo $FOO"
     ],
     "env": {
-      "HELLO": "WORLD",
-      "HOME": null
+      "FOO": "bar"
     },
     "name": "env step"
   },
+  {
+    "cmd": [
+      "bash",
+      "-c",
+      "echo $FOO"
+    ],
+    "env": {
+      "FOO": "[START_DIR]/pants:[START_DIR]/shirt:bar"
+    },
+    "name": "env step with prefix and suffix"
+  },
+  {
+    "cmd": [
+      "bash",
+      "-c",
+      "echo $FOO"
+    ],
+    "env": {
+      "FOO": "[START_DIR]/pants:bar"
+    },
+    "name": "env with path removal"
+  },
   {
     "cmd": [],
     "name": "nested"