Fix type inference error in tests on rustc beta

[yotamofek]

On the latest release of rustc on the beta channel, running cargo test fails:

error[E0283]: type annotations needed for `matrix::Matrix<T, base::dimension::Const<2>, base::dimension::Const<2>, array_storage::ArrayStorage<T, 2, 2>>`
   --> src/linalg/symmetric_eigen.rs:363:17
    |
363 |             let m = Matrix2::new_random();
    |                 ^   --------------------- type must be known at this point
    |
    = note: cannot satisfy `_: base::scalar::Scalar`
note: required by a bound in `construction::<impl matrix::Matrix<T, R, C, <default_allocator::DefaultAllocator as base::allocator::Allocator<T, R, C>>::Buffer>>::new_random`
   --> src/base/construction.rs:647:9
    |
639 |         pub fn new_random($($args: usize),*) -> Self
    |                ---------- required by a bound in this associated function
...
647 | impl<T: Scalar, R: DimName, C: DimName> OMatrix<T, R, C>
    |         ^^^^^^ required by this bound in `construction::<impl Matrix<T, R, C, <DefaultAllocator as Allocator<T, R, C>>::Buffer>>::new_random`
help: consider giving `m` an explicit type, where the type for type parameter `T` is specified
    |
363 |             let m: matrix::Matrix<T, base::dimension::Const<2>, base::dimension::Const<2>, array_storage::ArrayStorage<T, 2, 2>> = Matrix2::new_random();
    |                  +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

error[E0283]: type annotations needed
   --> src/linalg/symmetric_eigen.rs:364:23
    |
364 |             let m = m * m.transpose();
    |                       ^ cannot infer type for struct `matrix::Matrix<_, base::dimension::Const<2>, base::dimension::Const<2>, array_storage::ArrayStorage<_, 2, 2>>`
    |
note: multiple `impl`s satisfying `matrix::Matrix<_, base::dimension::Const<2>, base::dimension::Const<2>, array_storage::ArrayStorage<_, 2, 2>>: Mul<matrix::Matrix<_, base::dimension::Const<2>, base::dimension::Const<2>, array_storage::ArrayStorage<_, 2, 2>>>` found
   --> src/base/ops.rs:458:9
    |
458 | /         impl<T, R: Dim, C: Dim, S> $Trait<T> for Matrix<T, R, C, S>
459 | |             where T: Scalar + $bound,
460 | |                   S: Storage<T, R, C>,
461 | |                   DefaultAllocator: Allocator<T, R, C> {
    | |______________________________________________________^
...
509 |   componentwise_scalarop_impl!(Mul, mul, ClosedMul; MulAssign, mul_assign);
    |   ------------------------------------------------------------------------ in this macro invocation
...
607 | / impl<T, R1: Dim, C1: Dim, R2: Dim, C2: Dim, SA, SB> Mul<Matrix<T, R2, C2, SB>>
608 | |     for Matrix<T, R1, C1, SA>
609 | | where
610 | |     T: Scalar + Zero + One + ClosedAdd + ClosedMul,
...   |
613 | |     DefaultAllocator: Allocator<T, R1, C2>,
614 | |     ShapeConstraint: AreMultipliable<R1, C1, R2, C2>,
    | |_____________________________________________________^
    = note: this error originates in the macro `componentwise_scalarop_impl` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0283`.

I used cargo-bisect-rustc and got pointed to this PR: rust-lang/rust#119989
It seems like the author of the PR knew it might cause some regressions, but they were deemed acceptable.
This means that nalgebra's CI will break on May 2nd, when 1.78 gets promoted to stable.
This small change fixes the regression.

[Ralith]

Thanks!