The arguments given to `find_path()` and `find_library()` either matched the
default values, were redundant, or were better fixed by setting
`CMAKE_PREFIX_PATH` to point at the popt install location in the cmake cmdline.

The POPT_ROOT_DIR location hint has also been removed, but note that cmake
3.12 adds support for using `<PackageName>_ROOT` as the first location hint by
default.

In CMakeLists.txt, explicity execute scripts with bash on WIN32 platforms (it
doesn't understand `#!/bin/sh`). Execute test scripts with the full rdiff path
expanded from `$<TARGET_FILE:rdiff>` as the first argument instead of
`${CMAKE_CURRENT_BINARY_DIR}`.

Update tests/testcommon.sh to set `RDIFF` instead of `bindir` from $1, and use
`${RDIFF}` as the rdiff binary in all tests.

Change rdiff_bad_option.sh to use testcommon.sh and use $RDIFF to execute the
rdiff binary. Make it use the testcommon.sh created tmpdir and use set -x to
trace only the interesting cmds instead of the whole script.

The $<TARGET_FILE:rdiff> is always the full path of the generated binary,
including the .exe extension on Windows. The ${CMAKE_CURRENT_BINARY_DIR} is
usually the directory where the generated binary is, but multi-configuration
generators (Visual Studio, Ninja) add a per-build-type directory (Release,
Debug, etc) and put them in there.

This means ${CMAKE_CURRENT_BINARY_DIR} doesn't give you an easy way to find
the binary, and doesn't tell you if it has a .exe extension. Passing in the
full binary path solves all these problems. Fortunately the tests only need
the rdiff binary.

This is important to ensure that git doesn't apply "\n" -> "\r\n" text
conversions on checkout for Windows platforms. Changing the data files like
this breaks the tests that use them.