trim-range: Stress the call-set parser just a tiny bit more.

We specify an equivalent range, but in a slightly more complex way for
better testing.
Fix test to correctly use cmake feature to find python executable

On my system, at least, there was no ${python} variable set, so the
tests for PIL were incorrectly failing, (such that many tests were not
being run at all).
cli: Add test for "apitrace trim" with both --calls and --frames

This case was triggering a bug in "apitrace trim" where it would
incorrectly bail out after going past the last specified call (or
frame) without noticing that there were still further frames (or
calls) to be processed.
Rework testing of trimming of operations with no side effects

The previous test was bogus. The only call it trimmed was glxSwapBuffers
which is actually invalid, (this call definitely does have side effects,
but can easily be trimmed if desired with --trim-spec=drawing).

The new testing here uses a trace from a new program, (source
included), called few-side-effects which simply makes various calls
that do not have side effects, (such as glGetIntegerv and
glIsEnabled).

Then, the testing exercises the two independent mechanisms available
for trimming calls without side effects:

    trim --prune

	Trims calls within the user-specified range without side
	effects.

    trim --deps --trim-spec=no-side-effects

	Trims calls outside the user-specific range without side
	effects.

Three new tests are added here, one for each of the above cases and
one for both combined, (prune, trim, and prune-and-trim).