bug#44712: Inconsistent directory lookup of executable in dejagnu.exp/host_execute

[Robert Menteer]

If you call ‘host_execute /fullpath/testsuite/unit-test/unit’, host_execute can find the unit test but not execute it.

If you call ‘host_execute /testsuite/unit-test/unit’, host_execute can’t find the unit test (but could execute it).

If you call ‘host_execute testsuite/unit-test/unit’, host_execute works.

This is because the lookup of the executable is done using:

if {![file exists ${executable}]} 

And the executing is done using:

spawn -noecho "./$executable" ${params}

Please make the usage of $executable consistent and document how host_execute should be called.