GoogleTest: Add working directory to gtest_discover_tests error message

If the working directory didn't exist, the old message just says:

    Error running test executable.

        Path: '<Path to test executable>'
        Result: No such file or directory
        Output:

This leads the user to the conclusion that the test executable doesn't
exist, which isn't true.  Make the true cause visible by reporting the
working directory in the error message.

Modules/GoogleTestAddTests.cmake

@@ -112,6 +112,7 @@ function(gtest_discover_tests_impl)
     message(FATAL_ERROR
       "Error running test executable.\n"
       "  Path: '${path}'\n"
+      "  Working directory: '${_TEST_WORKING_DIR}'\n"
       "  Result: ${result}\n"
       "  Output:\n"
       "    ${output}\n"

Tests/RunCMake/GoogleTest/GoogleTest-discovery-POST_BUILD-timeout-build-stdout.txt

@@ -2,6 +2,7 @@
 ( *|[0-9]+>)  Error running test executable.
 ?( *|[0-9]+>)
 ( *|[0-9]+>)    Path: '.*discovery_timeout_test(\.exe)?'
+( *|[0-9]+>)    Working directory: '[^']*/Tests/RunCMake/GoogleTest/GoogleTest-discovery-timeout'
 ( *|[0-9]+>)    Result: Process terminated due to timeout
 ( *|[0-9]+>)    Output:
 ( *|[0-9]+>)     +

Tests/RunCMake/GoogleTest/GoogleTest-discovery-PRE_TEST-timeout-test-stderr.txt

@@ -2,6 +2,7 @@ CMake Error at .*GoogleTestAddTests.cmake:[0-9]+ \(message\):
 [ \t]*Error running test executable.
 +
 [ \t]*Path: '.*discovery_timeout_test(\.exe)?'
+[ \t]*Working directory: '[^']*/Tests/RunCMake/GoogleTest/GoogleTest-discovery-timeout'
 [ \t]*Result: Process terminated due to timeout
 [ \t]*Output:
 [ \t]*timeout.