Add a test for environment variables.

This adds basic support for environment variables to the test harness,
and adds a simple test.

This tests the bug found in https://github.com/CraneStation/wasi-libc/pull/159.

tests/general/env-absent.c

@@ -0,0 +1,7 @@
+#include <stdlib.h>
+#include <stdio.h>
+
+int main(void) {
+    printf("HELLO = %s\n", getenv("HELLO"));
+    return 0;
+}

tests/general/env-absent.c.stdout.expected

@@ -0,0 +1 @@
+HELLO = (null)

tests/general/env.c

@@ -0,0 +1,7 @@
+#include <stdlib.h>
+#include <stdio.h>
+
+int main(void) {
+    printf("HELLO = %s\n", getenv("HELLO"));
+    return 0;
+}

tests/general/env.c.env

@@ -0,0 +1 @@
+HELLO=hello

tests/general/env.c.stdout.expected

@@ -0,0 +1 @@
+HELLO = hello

tests/testcase.sh

@@ -41,9 +41,16 @@ else
   stdin="/dev/null"
 fi
 
+# Determine any environment variables to set.
+if [ -e "$input.env" ]; then
+    env=$(sed -e 's/^/--env /' < "$input.env")
+else
+    env=""
+fi
+
 # Run the test, capturing stdout, stderr, and the exit status.
 exit_status=0
-"$runwasi" "$wasm" \
+"$runwasi" $env "$wasm" \
     < "$stdin" \
     > "$stdout_observed" \
     2> "$stderr_observed" \