Add testcase for #5930.

libraries/base/tests/System/T5930.hs

+import Control.Monad (when)
+import Data.Maybe (isJust)
+import System.Environment (lookupEnv)
+
+main :: IO ()
+main = do
+    term <- lookupEnv "PATH"
+    when (isJust term) $ putStrLn "Got PATH"
+    fish <- lookupEnv "One fish, two fish, red fish, blue fish"
+    print fish

libraries/base/tests/System/T5930.stdout

+Got PATH
+Nothing

libraries/base/tests/System/all.T

@@ -2,6 +2,7 @@
 test('exitWith001', exit_code(42), compile_and_run, [''])
 test('getArgs001',  normal, compile_and_run, [''])
 test('getEnv001',   normal, compile_and_run, [''])
+test('T5930',   normal, compile_and_run, [''])
 
 test('system001',   if_platform("i386-unknown-mingw32", expect_fail), \
 	compile_and_run, [''])