remove awk dependency in test suite

I'm on NixOS, so awk isn't in /usr/bin. I can search for it with
/usr/bin/env, but then I can't use -f in the shebang line.

tests/Cross-shell test support/.test-run-by-specified-shell/With -s, a non-sh test script should be invoked directly.

@@ -1,5 +1,3 @@
-#!/usr/bin/awk -f
-
-# This script will only succeed if it is indeed processed by awk.
-
-BEGIN { print "ok" }
+#!/usr/bin/env true
+true will processed the contents of this script, but that
+means that nothing will happen and the script will exit 0 

tests/Cross-shell test support/The -s option should invoke tests with the specified shell.

@@ -3,6 +3,5 @@
 # Tests the `-s <shell> option, which invokes shebang-less and sh-shebang-line test scripts with the specified shell (for testing *sourced* shell code).
 
 which bash >/dev/null || { echo "Cannot test -s option: bash cannot be located." >&2; exit 2; }
-which /usr/bin/awk >/dev/null || { echo "Cannot test -s option: /usr/bin/awk not found." >&2; exit 2; }
 
 ../../urchin -s bash ./.test-run-by-specified-shell