Improve test cases that use objdump

Improve test cases that use objdump

- Added expect_objdump_contains and expect_objdump_not_contains utility
  functions to the main test framework, making objdump_cmd and grep_cmd
  (renamed to objdump_grep_cmd) internal utility functions.
- Removed strange support for the $HOST_OS_WINDOWS || $HOST_OS_CYGWIN
  case in objdump_grep_cmd. Let’s re-add it later if and when it becomes
  a problem.
- objdump_grep_cmd now greps for literal strings, thus no longer
  interpreting $PWD as a regex.
- Made test cases use expect_objdump_(not_)contains instead of
  “objdump_grep_cmd | grep_cmd”.