[context] Add path prefix/suffix manipulation.

recipe_modules/context/tests/env.expected/basic.json

Index: recipe_modules/context/tests/env.expected/basic.json
diff --git a/recipe_modules/context/tests/env.expected/basic.json b/recipe_modules/context/tests/env.expected/basic.json
index 2756a903db08fbd3d619be5bba298a967ba82aae..3627af6b934513b5d170c7cfc92b232968bb87b8 100644
--- a/recipe_modules/context/tests/env.expected/basic.json
+++ b/recipe_modules/context/tests/env.expected/basic.json
@@ -27,6 +27,50 @@
     },
     "name": "with 2 envs"
   },
+  {
+    "cmd": [
+      "bash",
+      "-c",
+      "echo $FOO"
+    ],
+    "env": {
+      "FOO": "bar"
+    },
+    "name": "env step"
+  },
+  {
+    "cmd": [
+      "bash",
+      "-c",
+      "echo $FOO"
+    ],
+    "env": {
+      "FOO": "foo:<FOO>:baz"
+    },
+    "name": "env step augmented"
+  },
+  {
+    "cmd": [
+      "bash",
+      "-c",
+      "echo $FOO"
+    ],
+    "env": {
+      "FOO": "[START_DIR]/pants:[START_DIR]/shirt:foo:<FOO>:baz"
+    },
+    "name": "env step with prefix"
+  },
+  {
+    "cmd": [
+      "bash",
+      "-c",
+      "echo $FOO"
+    ],
+    "env": {
+      "FOO": "[START_DIR]/shirt"
+    },
+    "name": "env with default value"
+  },
   {
     "name": "$result",
     "recipe_result": null,