Fix bug when last argument is a function with multiple return values

[LandonTClipp]

Description

There are two bugs fixed in this commit:

1. As noted in issue Variadic function issues.  #766, mockery failed to generate a mock because it incorrectly considered the return values themselves to be variadic, which is impossible. The fix is to always consider return tuples non-variadic.
2. The .EXPECT() struct had an incorrect signature when the argument was a variadic function. The reason was because it triggered off the presence of "..." in the type string instead of triggering off the variadic-ness of the outer function. The fix was to simply use the variadic flag to determine if the last argument should be variadic.

Fixes #766

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Version of Golang used when building/testing:

• 1.11
• 1.12
• 1.13
• 1.14
• 1.15
• 1.16
• 1.17
• 1.18
• 1.19
• 1.20