testsuite: check for ill-formed kcmdline

testsuite: check for ill-formed kcmdline

Commit ca3bf5d47cec ("iommu/amd: Introduces ivrs_acpihid kernel parameter")
in the kernel introduced an ill-formed kernel parameter, ivrs_acpihid.
The problem is that it may have a dot on the key side:
"ivrs_acpihid[00:14.5]=AMD0020:0". This could potentially trip our
parser of module options, but right now it's working as intended: the
only thing that happens is that after deciding "ivrs_acpihid[00:14" is a
module name, it will fail the underscores() routine and the option will
be ignored from the kmod pov (not kernel's pov since that driver parsers
the kernel command line by itself).