CI: test validation2 dataset at fortran_mpi's CI

[gxyd]

Description

the only different between validation and validation2 dataset is it sets the value of the read variable epscg (located at testsuite/validation/input/pot3d.dat's namelist
in validation dataset is 1e-12, while in validation2 dataset
it is 1e-8

This test set is being tested only with the possible purpose that the testsuite validation2 runs faster than validation dataset with GFortran and LFortran compilers both.

I'll first check whether all the CI tests pass with validation2 or not, just like they currently pass with validation dataset. Once it does pass the tests, I'll post the timings difference between running validation2 vs validation dataset.

NOTE: this PR isn't intended to be merged at all currently.

[gxyd]

It passes all the tests, but we can't reduce epscg further (i.e. for epscg=1e-7, it fails the tests with GFortran) with the current setup (for now atleast), I'll try to see if any other variable in pot3d.dat can be modified to reduce the time spent running POT3D with validation test

[gxyd]

This is the only point of difference between validation testsuite (which has epscg=1.e-12) and validation2 introduced in this PR.

[gxyd]

On my macbook air M4 machine with 16GB RAM (10 cores), with MPICH

MPI Ranks	Validation Dataset (seconds)	Validation2 (this branch) Dataset (seconds)
1	63.027569	47.466790
2	34.250361	26.317204
4	23.318231	16.478486

[certik]

I don't know what this is doing, but it doesn't seem to break anything either.

[gxyd]

I don't know what this is doing, but it doesn't seem to break anything either.

epscg acts as an epsilon for CG solver (Conjugate gradient) which is used in POT3D, by using a larger epscg (i.e. now 1e-8 instead of 1e-12), we converge rather quickly while still getting the same values for BP**2 etc, as this converges quickly, hence it runs faster.

We'll now test both with validation and validation2 at fortran_mpi's CI. I'll make further modifications to validation2 in further PR's, which will intend to further reduce the time taken to run POT3D program.

[gxyd]

The tests pass, so I'm merging this.