[tests] Add testltp program to enable testing with newer LTP versions

[efu39]

Description of the changes

This is a draft PR, to get input if there is interest to merge it.

With the LTP 1909cd5675 change, the command gramine-{direct|sgx} <TEST_BINARY> does not work with LTP releases after 20220527, failing with “TFAIL: Main test process might have exit”
The commit introduces testltp program, allowing newer versions of LTP to be tested using:

gramine-{direct|sgx} testltp <TEST_BINARY>  

Additionally, testing with pytest can be done by setting the LTP_ENV variable, for example:

 LTP_ENV=testltp python3 -m pytest -v -k chmod01
 LTP_ENV=testltp make [SGX=1] regression

The change enables testing with newer LTP versions for development purposes only. The LTP version in use remains unchanged.

How to test this PR?

For the current LTP version, LTP test behave as before and can be run with:make [SGX=1] regression
Tested with LTP version 20240930 using: LTP_ENV=testltp make [SGX=1] regression

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This change is 

[kailun-qin]

@efu39: Can we discuss this in the next core Gramine meeting?

Reviewable status: 0 of 4 files reviewed, all discussions resolved, not enough approvals from maintainers (2 more required), not enough approvals from different teams (2 more required, approved so far: )