@student_md This pdetest result is actually a list, not a matrix. The solution is applied (i.e. differentiated and then plugged into) the list of equations, that is [PDE, BCs, ICs], where BCs is a sequence of two equations in this example.
If you don't need the multiplicity info, use sets instead of lists. Simply replace the square brackets by curly braces then. However, if any non-zero result occurs, lists are helpful in pinpointing the equation that led to that result.
Please look up the ?pdetest help page for more details.
Side remarks, unrelated: pdsolve and pdetest are top-level commands, hence loading PDEtools is unnecessary here.
And helping pdsolve with the multiplicative ansatz (by providing one of the two factors) leads to the PDE (not the full IBVP!) being solved:
However, that requires too much knowledge about the solution, so I would see this more as an observation rather than a valid workaround.